The antitumor activity of spicamycin analogue SPM VIII against human stomach, breast, lung, colon and esophageal cancers was compared to that of mitomycin C (MMC) in the human tumor-nude mice xenograft model. Comparative studies of SPM VIII given i.v. at 6 mg/kg/day daily for 5 days and MMC given i.v. at 6.7 mg/kg on day 1 revealed that the antitumor spectrum of SPM VIII showed a different pattern from that of MMC and that SPM VIII caused tumor mass reductions in more tumors than did MMC in colon cancers (4/12 versus 1/11). In addition to this study, a comparative study of SPM VIII given i.v. at 12 mg/kg/day 8 times at 3- or 4-day intervals and 5'-deoxy-5-fluorouridine (5'-DFUR) given po at 185 mg/kg/day 5 days per week for 4 weeks showed that SPM VIII had the highest effect on SC-9 human stomach cancer and COL-1 human colon cancer among the 3 compounds, resulting in a significant reduction of tumor mass. Although other pharmacological studies are in progress, these results suggest that SPM VIII might be a novel antitumor compound effective for human cancers including cancer of the digestive organs.