The anti-inflammatory drug indomethacin was tested for antitumor activity against transplantable mouse colon adenocarcinoma 38 (colon 38). Groups of BDF1 mice (C57BL/6 X DBA/2) were given intraperitoneal injections of this drug beginning on the 6th day after subcutaneous implantation of the tumor and continued for 4 to 8 days. In other groups of mice, identical treatment was delayed until the 16th day after implantation of the tumor. The higher antitumor activity against colon 38 was obtained with earlier initiation of treatment, indicated by decreased growth of the tumor and increased life span of the host. The later initiation of the treatment produced less antitumor activity. The antitumor activity was, however, less than that of 5-fluorouracil, which was used as a positive control drug. The two drugs in combination produced few advantages over 5-fluorouracil alone using the dose schedule designed in the present experiment. Indomethacin treatment significantly reduced prostaglandin E and F levels in the tumor tissue, but 5-fluorouracil did not. It seems likely that the inhibition of prostaglandin biosynthesis by indomethacin underlies the antitumor effect of this drug on colon 38.