The authors studied the effects of the active ingredients contained in 43 pesticides and insecticides on dairy cultures and the suitability of milk contaminated in vitro with selected organophosphorus insecticides. The acid-forming property of yoghurt, kefir, butter and cheese cultures, the clotting property of milk and the ripening process of Camembert were used as testing criteria. The herbicides 2-methyl-4,6-dinitrophenol (DNOC), 2,4, 5-trichlorophenol, 2-methyl-4-chlorophenoxyacetic acid (MCPA), 4-(2-methyl-4-chlorophenoxy) butyric acid (MPCB), 2,4-dichlorophenoxyacetic acid (2,4-D), 2,2-dichloropropionic acid (dalapon), the chlorinated insecticide heptachlorepoxide and the fungicide pentachlorophenol exerted inhibitory effects when applied at concentrations of more than 100 p.p.m., whereas most of the organophosphorus and chlorinated insecticides under investigation produced no detectable effects on the culture activity when used at concentrations up to 100 mg/kg. The clotting property of milk was not impaired by the pesticides and insecticides at concentrations up to 100 mg/kg; the same was true of the ripening process of Camembert.