Ten xenograft lines of human colonic and rectal carcinomas have been established in immune-suppressed mice. Mice bearing tumours in the 2nd to 6th passage were treated with maximum tolerated doses of 8 chemotherapeutic agents and tumour growth delay was estimated in terms of the number of volume doubling times gained by the treatment. The average response corresponded to a delay of only about 0.5 doubling times, but some tumour lines showed a good response to some agents. Twenty-three out of about 700 treated tumours failed to regrow. Statistical analysis showed no consistent difference in sensitivity among the tumour lines, but melphalan, 5-fluorouracil and hexamethylmelamine stood out as the most effective agents. A study of two-drug combinations showed that their order of administration had little effect on response. Only 4 of the patients who donated the xenografts were treated with chemotherapy, but among these there was some evidence that response in the xenografts correlated with response in the patient.