The inhibitory effect of Antineoplaston A-10 was tested on the growth curve of human breast cancer (R-27) serially transplanted to athymic mice and on the cell growth of human hepatocellular carcinomas (KMCH-1, KYN-1, KIM-1). Approximately 1.25% of Antineoplaston A-10 in the regular mouse diet (CE2) inhibited the growth curve significantly after 35 days treatment (p less than 0.05). Seventy milligrams of daily intraperitoneal administration of Antineoplaston A-10 Injection also inhibited the growth curve of R-27 after 52 days treatment (p less than 0.01). Histological study of tumor showed no essential difference in structure but significant decrease in number of mitoses in the Antineoplaston A-10 treated groups. The cell growth of human hepatocellular carcinoma cell lines KMCH-1, KYN-1 and KIM-1 was inhibited by Antineoplaston A-10 Injection dose dependently.