A method for immunological determination of the concentration of a low-molecular-weight analyte in a liquid biological sample, the analyte of lower molecular weight being a non-peptidic amine compound with a molecule weight of less than 500 Dalton or a peptidic compound with a reactive amino group and of a molecular weight of less than 1500 Dalton, comprising the following method steps: adding to a sample with said low-molecular-weight analyte a stoichiometric excess of derivatizing agent which has a chemically reactive group quantitatively reacting with an amino group of said low-molecular-weight analyte and which derivatizing agent has within a distance of less than 12 carbon bonds to said chemically reactive group a group that has π electrons or π ligand orbitals and selected from compounds of the general formulae (Ia) or (IIa): wherein R is a peptidic residue or a hydrocarbon having up to 40 carbon atoms which may comprise hydroxy or keto groups and optionally N, P, O, or S; reacting said analyte in said sample with said derivatizing agent, optionally followed by a suitable stop of the reaction with said derivatizing reagent, and obtaining a derivatized analyte in said sample; providing antibodies or antibody fragments, made against and specific for said derivatized analyte, and wherein the contact region between the hypervariable domains of the variable chains of said antibody and the derivatized analyte contains a group with π electrons or π ligand orbitals as well as the amino group after its reaction with a reactive group of said derivatizing agent; contacting said sample with said derivatized analyte with said specific antibodies or antibody fragments and determining the amount of derivatized analyte in said sample by binding analysis, and determining the concentration of the low-molecular-weight analyte in said sample by comparison with a series of standards, wherein the analyte is a biogenic amine and selected from a group comprising asymmetric dimethyl arginine (ADMA), symmetric dimethyl arginine (SDMA), gamma-amino butyric acid (GABA), L-glycine, L-tryptophan, amines of the kynurenine pathway, L-arginine, homocysteine, citrulline, D-serine, cysteine, methionine, carnitine, phosphoserine, sulfocysteine, argininosuccinate, hydroxyproline, aspartate, phosphor ethanolamine, sarcosine ( N -methylglycine), carnosine (ß-alanyl-L-histidine), anserine (ß-alanyl-N-methyl-L-histidine), 1,3-methylhistidine, alpha-amino adipic acid, methoxytryptamine, beta-alanine, proline, ethanolamine, beta-aminoisobutyric acid, L-allocystathionine-A, L-cystathionine, cystine, alloisoleucine, DL-hydroxylysine, DL-allohydroxylysine, und monocystine; serotonin, histamine; glutamine; catecholamines, dopamine, metanephrine, nor-metanephrine; cholinergic amines, adrenaline, noradrenaline; oligopeptides and small peptide hormones having up to 12 amino acids, nonapeptides, vasopressin, oxytocin; decapeptides, angiotensin-I; octapeptides, angiotensin-II; T3 (triiodothyronine), und T4 (thyroxine). adding to a sample with said low-molecular-weight analyte a stoichiometric excess of derivatizing agent which has a chemically reactive group quantitatively reacting with an amino group of said low-molecular-weight analyte and which derivatizing agent has within a distance of less than 12 carbon bonds to said chemically reactive group a group that has π electrons or π ligand orbitals and selected from compounds of the general formulae (Ia) or (IIa): wherein R is a peptidic residue or a hydrocarbon having up to 40 carbon atoms which may comprise hydroxy or keto groups and optionally N, P, O, or S; reacting said analyte in said sample with said derivatizing agent, optionally followed by a suitable stop of the reaction with said derivatizing reagent, and obtaining a derivatized analyte in said sample; providing antibodies or antibody fragments, made against and specific for said derivatized analyte, and wherein the contact region between the hypervariable domains of the variable chains of said antibody and the derivatized analyte contains a group with π electrons or π ligand orbitals as well as the amino group after its reaction with a reactive group of said derivatizing agent; contacting said sample with said derivatized analyte with said specific antibodies or antibody fragments and determining the amount of derivatized analyte in said sample by binding analysis, and determining the concentration of the low-molecular-weight analyte in said sample by comparison with a series of standards, wherein the analyte is a biogenic amine and selected from a group comprising asymmetric dimethyl arginine (ADMA), symmetric dimethyl arginine (SDMA), gamma-amino butyric acid (GABA), L-glycine, L-tryptophan, amines of the kynurenine pathway, L-arginine, homocysteine, citrulline, D-serine, cysteine, methionine, carnitine, phosphoserine, sulfocysteine, argininosuccinate, hydroxyproline, aspartate, phosphor ethanolamine, sarcosine ( N -methylglycine), carnosine (ß-alanyl-L-histidine), anserine (ß-alanyl-N-methyl-L-histidine), 1,3-methylhistidine, alpha-amino adipic acid, methoxytryptamine, beta-alanine, proline, ethanolamine, beta-aminoisobutyric acid, L-allocystathionine-A, L-cystathionine, cystine, alloisoleucine, DL-hydroxylysine, DL-allohydroxylysine, und monocystine; serotonin, histamine; glutamine; catecholamines, dopamine, metanephrine, nor-metanephrine; cholinergic amines, adrenaline, noradrenaline; oligopeptides and small peptide hormones having up to 12 amino acids, nonapeptides, vasopressin, oxytocin; decapeptides, angiotensin-I; octapeptides, angiotensin-II; T3 (triiodothyronine), und T4 (thyroxine).