The invention comprises products containing formaldehyde reversibly bonded (i.e. chemically bonded in a reversible manner so that it is capable of being split off) to a uracil of the formula <FORM:0923108/IV(a)/1> (wherein R and R1 are hydrogen atoms or alkyl groups of at most 4 carbon atoms, R2 is an alkyl group of at most 4 carbon atoms or an amino group, R3 is a hydrogen atom or an amine group which may be substituted by one or more alkyl groups of at most 4 carbon atoms, alkenyl groups of 3 or 4 carbon atoms, aryl groups, aliphatic acyl groups, omega-carboxy-acyl groups, aromatic aminoacyl groups or aromatic aminosulphonyl groups, X is an oxygen or sulphur atom and n is zero or 1; and wherein at least one amino or imino group containing a replaceable hydrogen atom is present in the ring or in the substituent in the 4- or 5-position) and their preparation by reacting formaldehyde or formaldehyde-yielding compound with the appropriate substituted uracil at neutral or alkaline pH.  The products preferably contain between 10 and 30% by weight of formaldehyde, and the reaction is preferably effected in aqueous solution.  It is stated that when, as is preferred, at least one mol of formaldehyde is used for every amino and/or imino group containing a replaceable hydrogen atom, then the quantity of reversibly bonded formaldehyde present in the product is constant.  Detailed examples are provided.  N-(1,3-Dimethyl-4- amino-uracilyl-5)-maleic acid monoamide is prepared from 1,3-di-methyl-4,5-diaminouracil and maleic anhydride. 1,3-Dimethyl-4- amino-5-N1- allyl-thioureido-uracil is prepared from 1,3-dimethyl-4,5-diaminouracil and allyl mustard oil. 3-Methyl-4-amino-5-N1-phenylthioureido-uracil is similarly prepared.ALSO:Disinfectant compositions for medical and non-medical use comprise a product containing formaldehyde reversibly bonded (i.e. chemically bonded in a reversible manner so that it is capable of being split off) to a uracil of the formula <FORM:0923108/VI/1> (wherein R and R1 are hydrogen atoms or alkyl groups of at most 4 carbon atoms, R2 is an alkyl group of at most 4 carbon atoms or an amino group, R3 is a hydrogen atom or an amino group which may be substituted by one or more alkyl groups of at most 4 carbon atoms, alkenyl groups of 3 or 4 carbon atoms, aryl groups, aliphatic aryl groups, omega-carboxy-acyl groups, aromatic aminoacyl groups or aromatic aminosulphonyl groups, X is an oxygen or sulphur atom and n is zero or 1; and wherein at least one amino or imino group containing a replaceable hydrogen atom is present in the ring or in the substituent in the 4- or 5-position) and inert diluents or carriers.  They may take the form of tablets, lozenges, pastilles, solutions, tinctures, aerosol sprays, ointments, emulsions or powders and may also contain emulsifying, wetting and dispersing agents, fluorocarbons (e.g. difluorodichloromethane), and further substances having antimicrobial activity such as phenols and quaternary ammonium compounds.  The compositions are stated to provide reversibly bonded formaldehyde in a form suitable for administration to humans, and they may be used in formulations having a disinfectant or antiseptic effect for the treatment and prophylaxis of inflammatory diseases.