CPT-11 (irinotecan hydrochloride, 7-ethyl-10-[4-(piperidino)-1-piperidino] carbonyloxy-camptothecin) is a semisynthetic camptothecin derivative developed in Japan. The inhibitory activity of CPT-11 against human tumor colony-forming units from freshly explanted human tumors was explored using a soft agar cloning system. Final CPT-11 concentrations of 0.3-3.0 micrograms/ml were used for a 1 h exposure. At a concentration of 3.0 micrograms/ml antitumor activity was seen against colorectal, ovarian, nonsmall-cell lung, breast cancer and mesothelioma colony-forming units. CPT-11 should have activity against a broad spectrum of tumors in patients.