2',2'-Difluorodeoxycytidine (LY 188011, Gemcitabine) is a novel pyrimidine antimetabolite with promising activity in preclinical models for leukemia and solid tumors. Phase I clinical trials with the agent are ongoing. In order to better define types of tumors with clinical sensitivity to Gemcitabine (to help target phase II trials), we have studied the antitumor effects of this agent against a variety of freshly explanted human tumor specimens using an in vitro capillary soft agar cloning system. Final concentrations of 2.0-200 micrograms/ml were used for short-term (1 h) and continuous incubations experiments. Using a short-term incubation, 94/215 (44%) tumor specimens were evaluable for the determination of antitumor activity. The most common tumor types studied included colorectal, breast, non-small cell lung, ovarian cancer, kidney and melanoma. A concentration-dependent increase in the frequency of inhibited tumor specimens was noted (2 micrograms/ml: 6/94 specimens, 20 micrograms/ml: 13/94 specimens, 200 micrograms/ml:33/94 specimens; p less than 0.0001). A similar increase in tumor growth inhibition was found using a continuous incubation (2 micrograms/ml: 0/14 specimens, 20 micrograms/ml: 1/14 specimens, 200 micrograms/ml: 7/14 specimens; p less than 0.001). We conclude that Gemcitabine is an active antitumor agent against tumor colony forming units from a variety of human malignancies if sufficiently high concentrations can be achieved. The agent should be evaluated for Phase II clinical activity against those tumor types.