Since the cytotoxic effect of ionizing radiation increases in faster proliferating cell populations, the effects of a mild trypsin and bromelain treatment on the growth rate of Ehrlich ascites tumor (EAT) cells cultured in vitro were studied. A continuous exposure to proteinase started at the time of cell plating caused a temporary block of DNA synthesis that was followed by an accelerated growth rate 48 h later (approximately 1.5-fold). EAT cells exposed to bromelain and trypsin after completed adaptation to the substratum demonstrated a similar increase of growth 2 days after the beginning of the enzyme treatment. The acceleration of growth was also observed when exposure to proteinase was interrupted after 24 h but the stimulation effect was reversible and continued only 2 days. It is concluded that bromelain and trypsin are able to modify reversibly the growth rate of EAT cell population cultured in vitro.