Abstract: AT-290, p-bis (2-chloroethyl) aminomethyl phenylalanine, synthesized at our Institutc, is a new derivative of nitrogen mustard. Animal experiments showed that AT-290 had a significant antitumor activity. The results on toxicity and antitumor activity were as follows: 1. The acute and subacute intraperitoneal LD₅₀ for mice were 4 and 1.6 mg/kg/day respectively. The subacute oral LD₅₀ in mice was 11 mg/kg/day and the subacute intraperitoneal LD₅₀ in rats was 1 mg/kg/day. 2. In 3 dogs, intravenous injections of 75 γ/kg/day of AT-290 for 10 days produced no remarkable influence on the general behavior, erythrocyte, leukocyte and platelet counts, bleeding and coagulation time, urinalysis and EKG, but caused a significant lymphocytopenia in the peripheral blood. In another 6 dogs, AT-290 at the dosages of 150 and 300 γ/kg exhibited an inhibiting effect upon the bone marrow, as evidenced by a peripheral leukopenia and thrombopenia, delayed bleeding time, and hematopoitic depression in the bone marrow. The animals treated with 75 and 150 γ/kg gradually recovered after cessation of the drug, while those given 300 γ/kg died in 13—19 days. 3. AT-290 inhibited significantly the growth rate of Jensen sarcoma and fibrosarcoma M-57 in rats. Experiments on 3 ascites tumors of mice (Ehrlich ascites carcinoma, spindle cell sarcoma and mammary adenocarcinoma) showed that AT-290 significantly prolonged the survival time of the tumor-bearing mice.