The tumor incidence was studied in 4 groups of BALB/c mice kept under observation in the same experimental conditions as untreated controls in the course of bioassays of chemical carcinogenesis. Two parameters that reflect the variations of environmental factors were studied: body growth and survival rate. Their possible influence on tumor incidence was also considered. Fluctuations were observed in the incidence of different tumor types among the groups, but the level of significance was reached only when comparing the incidence of hepatomas, adrenal cortex tumors and subcutaneous fibrosarcomas among males and the incidence of angiomas and angiosarcomas among females. No association was found between the variations of tumor incidence and body growth and survival rate, which remained constant among the groups.