There was assessed the frequency of inhibitory substances present in milk supplied to the milk plant in Olsztyn. There were used three standard microbiological methods and the Penzym test. There were carried out preliminary studies in respect to the sensitivity of microbiological tests used in Poland for the presence of anionic and non-anionic detergents in fresh milk. The findings indicated that the degree of contamination of milk by inhibitory substances was very high (up to 28.7% in winter). In milk produced from such products a several times increase of frequency of inhibitory substances was found compared with that found in fresh milk. Of three microbiological tests used in Poland only two, ie Polutest M and STD could be used to discover surface-active substances in milk. The Biolacta test proved to be of low sensitivity.