The article describes a colorimetric method for estimating formaldehyde using the Hantzsch reaction, which involves the synthesis of diacetylidi-hydrolutidine (DDL) from acetylacetone and formaldehyde in the presence of ammonium salts. The method is suitable for use with living biological material due to its mild conditions. The reaction produces a yellow color that can be quantified using a colorimeter, with a maximum extinction of 8000 at 412 mμ. The efficiency of the reaction is optimal at pH 5-5.6 and with specific concentrations of acetylacetone and ammonium acetate. The reagent is stable for at least two weeks and is less toxic to living material compared to other methods. The sensitivity of the method is comparable to chromotropic acid, and it has good specificity, with minimal interference from other substances. The article also discusses the preparation and properties of related compounds and the stability of the color produced. The method was developed for use in the Public Health Laboratory Service Committee on Formaldehyde Disinfection.The article describes a colorimetric method for estimating formaldehyde using the Hantzsch reaction, which involves the synthesis of diacetylidi-hydrolutidine (DDL) from acetylacetone and formaldehyde in the presence of ammonium salts. The method is suitable for use with living biological material due to its mild conditions. The reaction produces a yellow color that can be quantified using a colorimeter, with a maximum extinction of 8000 at 412 mμ. The efficiency of the reaction is optimal at pH 5-5.6 and with specific concentrations of acetylacetone and ammonium acetate. The reagent is stable for at least two weeks and is less toxic to living material compared to other methods. The sensitivity of the method is comparable to chromotropic acid, and it has good specificity, with minimal interference from other substances. The article also discusses the preparation and properties of related compounds and the stability of the color produced. The method was developed for use in the Public Health Laboratory Service Committee on Formaldehyde Disinfection.