A method of detecting the preovulatory LH peak in a biological sample obtained from mammals and comprising the following steps: a. fixing an anti-LH antibody on a test surface; b. contacting the test surface on which said anti-LH antibody is fixed with a buffer solution comprising 5 to 50 vol % of fetal calf serum; c. contacting said surface obtained in step (b) with a biological sample d. after step (c), rinsing the test surface in a washing solution; e. contacting the test surface rinsed in step (d) with a buffer solution of conjugate comprising an enzyme-coupled anti-LH antibody and from 5 to 50 vol % of fetal calf serum; f. after step (e), rinsing the test surface in a washing solution; and g. after step (f), contacting the test surface with a solution comprising a substrate of said enzyme, said substrate being selected from the group comprising 3,3',3,3',5,5'-tetramethylbenzidine (Membrane TMB), 4-chloro-1-naphtho/3,3'-diaminobenzidine tetrahydrochloride (CN/DAB), 3-amino-9-ethylcarbazole, 3,3-dimethoxybenzidine-o-dianisidine, 5-bromo-4-chloro-3'-indolyl phosphate p-toluidine salt (BCIP), a mixture of nitro-blue tetrazolium chloride and of 5-bromo-4-chloro-3'-indolyl phosphate p-toluidine salt (BCIP), the mixture Naphthol As-Mx phosphate and 4-chloro-2-methylbenzene diazonium salt (Fast red TR salt), the mixture Naphthol As-Mx phosphate and the diazotized salt of 4'-amino-2',5'-diethoxybenzanilide zinc chloride (Fast blue BB salt), 5-iodo-3-indolyl-β-D-galactopyranoside (Purple-Gal), 5-bromo-6-chloro-3-indolyl-β-D-galactopyranoside (Red-Gal), 6-chloro-3-indolyl-β-D-galactopyranoside (Rose-Gal), 5-bromo-3-indolyl-β-D-galactopyranoside (Blue-Gal), N-methylindolyl-β-D-galactopyranoside (Green-Gal). a. fixing an anti-LH antibody on a test surface; b. contacting the test surface on which said anti-LH antibody is fixed with a buffer solution comprising 5 to 50 vol % of fetal calf serum; c. contacting said surface obtained in step (b) with a biological sample d. after step (c), rinsing the test surface in a washing solution; e. contacting the test surface rinsed in step (d) with a buffer solution of conjugate comprising an enzyme-coupled anti-LH antibody and from 5 to 50 vol % of fetal calf serum; f. after step (e), rinsing the test surface in a washing solution; and g. after step (f), contacting the test surface with a solution comprising a substrate of said enzyme, said substrate being selected from the group comprising 3,3',3,3',5,5'-tetramethylbenzidine (Membrane TMB), 4-chloro-1-naphtho/3,3'-diaminobenzidine tetrahydrochloride (CN/DAB), 3-amino-9-ethylcarbazole, 3,3-dimethoxybenzidine-o-dianisidine, 5-bromo-4-chloro-3'-indolyl phosphate p-toluidine salt (BCIP), a mixture of nitro-blue tetrazolium chloride and of 5-bromo-4-chloro-3'-indolyl phosphate p-toluidine salt (BCIP), the mixture Naphthol As-Mx phosphate and 4-chloro-2-methylbenzene diazonium salt (Fast red TR salt), the mixture Naphthol As-Mx phosphate and the diazotized salt of 4'-amino-2',5'-diethoxybenzanilide zinc chloride (Fast blue BB salt), 5-iodo-3-indolyl-β-D-galactopyranoside (Purple-Gal), 5-bromo-6-chloro-3-indolyl-β-D-galactopyranoside (Red-Gal), 6-chloro-3-indolyl-β-D-galactopyranoside (Rose-Gal), 5-bromo-3-indolyl-β-D-galactopyranoside (Blue-Gal), N-methylindolyl-β-D-galactopyranoside (Green-Gal).