The cytostatic action of Propionibacterium granulosum was studied in a mouse sarcoma in vivo. Kinetic analysis of tumor cells 28 days after tumor implantation and systemic immunotherapy showed that the cell cycle time was identical in both treated and untreated tumors. P. granulosum treatment resulted in a marked prolongation of the S phase and shortening of the G1 phase of the cell cycle. A pronounced drop in the number of labeled interphases and the reduction of the growth fraction were observed in tumors obtained from mice given injection of P. granulosum. Cloning efficiency of tumor cells from P. granulosum treated animals was quantitatively similar to that of control animals and only differences in the size of lung colonies were observed.