Male Swiss mice with Ehrlich ascites tumor transplants were fed ad libitum yogurt mixed with drinking water; control mice were given only water. Both groups received regular mouse diet. Total tumorcell count and DNA content of cells were used as a measure of tumor growth. After 8 days, an average of 28% inhibition of cell proliferation was evident in the mice given yogurt. Daily cell counts and DNA determinations for 7 consecutive days showed rapid tumor growth in controls during the initial 6 days; thereafter growth reached a plateau. After the 2d day, significantly (P<0.05) lower cell counts and DNA values were evident in yogurt-fed mice. Inhibition of tumor cell proliferation reached a maximum at the end of the 3d day and thereafterremained the same through the 7th day.