Dolastatin 10 (1) is a potent antineoplastic pentapeptide. Novel dolastatin 10 analogs each modified at one of the constituent amino acid derivatives, were synthesized and their antitumoractivity was evaluated against P388 leukemia in mice. The structuralrequirements for antitumoractivity are discussed. Some of the analogs, 31c, 35c, 38b, and 50c showed excellent activity in vivo. Highly active 50c