A defined mixture of 25 chemicals that are often found in contaminated ground water was prepared as a water solution and studied for mutagenicity in bacteria, for prophage induction in bacteria, for palatability and effect on weight-gain in rats and mice, and for cytogenetic effects in bone marrow cells of rats and mice. In vitro tests for bacterial mutations and prophage induction were negative. A two-week exposure to the 25-chemical mixture in drinking water resulted in a concentration-related decrease in water consumption in male and female rats and mice. Concentration-related decreases in weight gain were observed in male and female mice; in rats, only the high dose groups showed decreased weight gains. The cytogenetic studies showed a small but significant increase in SCE in male mice and a similar weak effect on micronucleated polychromatic erythrocytes in the bone marrows of males and females. Also in bone marrow of male and female mice, an increase in mitotic index and a decrease in average cell generation time was observed. The %PCE in bone marrow were decreased in female mice only, while the %PCE in peripheral blood was increased in both sexes. In rats, the only effects observed in the cytogenetic studies were increased PCE frequencies in the peripheral blood of males and in the bone marrow of males and females.