Fresh surgical explants of solid tumors obtained from 84 patients were tested against the same chemotherapeutic agents in both the in vitro human tumor cloning (HTC) assay and the in vivo subrenal capsule (SRC) assay. Control growth adequate to meet evaluable assay criteria was obtained in 75 of 84 tumors tested in the SRC assay (89%) and in 33 of 79 tumors tested in the HTC assay (42%). Correlations between the 2 test systems were dependent upon the activity criteria established for each system. With activity criteria set at current drug screening levels as a decrease of .gtoreq. 50% in tumor colony-forming units for the HTC assay and a change in tumor size < -1.0 ocular micrometer unit for the SRC assay, 16% of the drugs tested were active in the SRC assay, and 7% were active in the HTC assay. Correlations of tumor response between the 2 assays were 29% for sensitive (2 of 7) and 83% for resistant (63 of 76). Of the 84 patients providing tumor tissue for assay, 17 had clinically evaluable disease and received chemotherapy providing information for retrospective analysis. A total of 23 SRC assay-clinical correlations and 10 HTC assay-clinical correlations were possible. The SRC assay was predictive of clinical sensitivity in 3 of 3 drug tests (100%) and of clinical resistance in 16 of 20 drug tests (80%). No HTC assay-clinical correlations were possible for sensitivity, but the HTC assay was predictive of clinical resistance in 10 of 10 drug tests (100%).