The effects of deprivation of oxygen on proliferation kinetics of in vitro grown Ehrlich ascites tumor cells were studied by means of flow cytometry. The flow cytometric results obtained by applying the BUdR-H 33258 technique showed that 6--8 hours after establishment of anaerobiosis cells lose their capacity for proliferation and accumulate in the late G1-phase; during this time late S cells enter the G2 compartment but do not divide while cells, which are in G2+M at the beginning of anaerobiosis divide and enter G1. Cell cycle distribution remained constant up to about 20 hours, thereafter G1-cells enter the S-phase. Up to 24 hours of anaerobiosis cell number does not change and viability of the cells was not severely affected. Recultivation under aerobic conditions of 12 hours anaerobically treated cells revealed, that the cytokinetic properties of G1 cells were not impaired, while S-phase cells after reaeration did not enter G2 but began again to synthesize DNA forming cells with a DNA content greater than 4c. S-phase cells are most sensitive to deprivation of oxygen.