Toxic action of hydrozine sulfate in rats and mice along with its influence on the mitotic activity of Walker carcino-sarcoma cells were studied. Single injections of the agent in high doses resulted in destruction of the animals during the first 2 days with no "delayed death" observed. Multiple injections caused a considerable loss of body weight. When given in toxic doses the agent did not influence hemopoiesis, but did affect sensitivity, drastically increasing the toxic action of ethanol or barbiturates. Administration of hydrazine sulfate in therapeutic doses decreased markedly the mitotic activity and increased the appearance of atypical mitotic cells.