A hypothalamic hormone--melanostatin H-L-Pro-L-Leu-NH2- and its 9 analogs were synthesized and their antitumor properties studied. Melanostatin caused a 52-72% inhibition of tumor growth (p less than 0.05) in mice bearing adenocarcinoma of the mammary gland Ca-755, cervical carcinoma CC-5 and melanoma B-16. Non-cytotoxic analogs containing D-leucine or L-lysine showed low activity. Among analogs containing sarcolysine stereomers, chlorphenacyl or chlorambucil, derivatives with L-sarcolysin exerted a high antitumor effect on Ca-755, CC-5, Lewis lung carcinoma, lymphoid leukemia L-1210, sarcoma-37, melanoma B-16 and S-91 (80-99% inhibition of tumor growth, p less than 0.05). L-sarcolysin alone had a higher effect on S-91 only (p less than 0.05). Antitumor effect of melanostatin is due to its amino acid sequences. Melanostatin analogs modified by L-phenylalanine retain their antitumor properties.