The antitumor efficacy of the cis-diamminedichloroplatinum (cisDDP) and 5 fluorouracil (5FU) combination was evaluated in a panel of eight human colorectal carcinoma xenografts. Tumors differed in origin (primary or metastatic), differentiation degree and chemotherapy treatment. Xenografts were treated with repeated i.v. injections of cisDDP, 5FU, or both drugs at 24-h interval. Compared with controls, cisDDP achieved a significant tumor growth inhibition in five out of eight tumor lines, and 5FU in four out of seven. One of two unresponsive tumor lines was significantly inhibited by the combination, that was also more effective than either drug alone (p less than 0.05) in one responsive xenograft. Comparing the effects of the combination according to which drug was administered first, lower drug doses were tolerated using the cisDDP-5FU sequence, but the antitumor effects were similar at equitoxic doses. These results indicate a potential therapeutic benefit of the cisDDP-5FU combination for colorectal carcinoma patients and show that toxicity of the combination is influenced by the drug sequence.