Approximately 800 amino acid derivatives have been synthesized and screened in order to evaluate their antitumor activity against various transplantable rat ascites hepatomas. Among them, N6,N9-bis(butyloxycarbonylaminomethyl)-L-citrulline (A-924) was found to be highly effective against various rat ascites hepatomas. A-924, when given orally, exhibited prolongation of survival of rats implanted intravenously with ascites hepatoma cells such as AH-44, AH-66, AH-130, AH-66F, or AH-41C.