The purpose of the study was to examine 7 yeast strains: Candida famata MI1a, C. intermediata BI2a, C. kefyr PII1b, C. sphaerica FII7a, Geotrichum penicillatum EII6a, Sacharomyces kluyveri BII3a and Yarrowia lipolytica PII6a, representing main species of the microflora occurring in Rokpol cheese. They were introduced aseptically to milk at 10⁵ cfu ml^-1 and incubated for 12 days. Their growth as well as their activity against milk components were evaluated. All the strains under investigation showed a different growth pattern in milk. The highest growth dynamics was exhibited by Y. lipolityca PII6a. On the 4^th day of incubation, its level reached of 1.0-1.4 × 10⁸ cfu ml^-1_, while C. famata MI1a and C. kefyr PI1b at the 8^th day reached only 1.9 × 10⁶ cfu ml^-1. A decrease in the pH value was observed in all the samples. Each yeast strain affected the changes in organic acid content of milk to a different extend. The growth of C. kefyr, C. sphaerica and C. intermediata was correlated with the accumulation of vast amounts of acetic acid, C. sphaerica FII7a, S. kluyveri BII3a and C. famata MI1a with propionic acid and Y. lipolytica with butyric acid. Yeast growth was accompanied with an intensive degradation of milk proteins and lipids. The highest rate of proteolysis of milk proteins was noted in the samples incubated with Y. lipolytica PII6a and C. sphaerica FII7a. The levels of nitrogen soluble at pH 4.6 in those samples were 30.2% and 28.4%, and free amino groups were 2612 and 2173 μMGly^.100mL^-1 of milk respectively. Those two strains also showed the highest ability to release free fatty acids (FFA) from triacylglycerols. On day 12 of incubation the FFA levels were 3320 and 2900 mg^.100 ml^-1, respectively. The dominant FFAs were: oleic and myristic acids.

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