A rapid autoradiographic technique for measuring the [3H]thymidine-labeling index of human solid tumors was adapted to assess the effect of anticancer drugs in vitro. The drugs tested were present unchanged or as metabolites in serum obtained from patients immediately posttreatment. In 15 patients, the drugs tested in vitro were also given in vivo. Tumor-labeling index fell significantly in 5 of 6 patients who later had objective clinical responses. Tumor-labeling index did not change significantly in 8 patients and rose significantly in 1 of 9 patients who lacked clinical response. If confirmed, this in vitro test may prove to be a useful method of predicting responsiveness of human solid cancers to chemotherapeutic agents.