The authors carried out experiment on 38 Syrian hamsters, treated with cyneb, tetracycline and chlorpromasine in groups for a period of tree days. They examined the changes in the cariotypes of the animals as well as the changes in the mytotic activity of bone-marrow cells. Data were based on the analysis of 3045 metaphasic plates and 190,000 cells. It was established that the examined substances possessed mutagenic effect on the Syrian hamster in doses applied in vivo. This effect was mostly manifected in the animals treated with cyneb, followed by chlorpromasine and tetracycline. The same chemical substances reduced the mytotic activity of the bone-marrow cells. The data of the experiments in vivo correlated positively with the results of the experiments in vitro of human lymphocytic cultures, treated with the indicated substances.