Mutagenic and carcinogenic properties of mildronate (3-[2.2.2.-trimethyl hydrazonium]-propionate) have been studied for its marked immuno-stimulating and anti-ischemic action. When dosage up to 1,000 mg/dish was used no reversal of base-pair substitution or frame shift in S.typhimurium was observed. In drosophila females treated with mildronate, mosaic patches were, on the average, as frequent as in control. Although chronic treatment of female mice B6D2F1, C3H and SHR with mildronate was not followed by any change in tumor incidence, mammary gland adenocarcinoma development was slightly inhibited in mice C3H and SHR. The latter effect was in correlation with the antigonadotropic one of the drug and was not determined by its estrogenous properties. This pointed to the absence of mutagenic and carcinogenic properties which was confirmed in two short-term and one chronic experiments on mice of the three lines.