Abstract: N-Formylsarcolysine (N-F) is an effective antitumor agent with a wide antitumor spectrum. The inhibitory effect of N-F on rat Yoshida sarcoma (both ascitic and solid forms), Walker carcino-sarcoma 256, mouse reticulosarcoma L₂, Krebs-2 ascites carcinoma, and ascites tumor L₁ showed that this compound was very active. The growth of Ehrlich carcinoma (both ascitic and solid forms) and spindle cell sarcoma B₂₂ were also inhibited by N-F. However, N-F has no marked effect on other mice tumors including sarcoma 180, sarcoma AK and melanoma Me. The single oral dose of LD₅₀/30 days in mice was 730±79 mg/kg, and that in rats was 700±79 mg/kg. The single intraperitoneal dose of LD₅₀/14 days in mice and rats were 152±7 mg/kg and 80±10 mg/kg respectively. The single intravenous dose of LD₅₀/30 days in mice was 155±10 mg/kg. The most common toxic effects of N-F were anorexia and leukopenia, but these were not serious and promptly disappeared when the drug was discontinued.