The same chemotherapeutic agents were tested against fresh surgical explants of solid tumors obtained from 50 patients using the in vivo subrenal capsule (SRC) assay and the in vitro succinate dehydrogenase inhibition (SDI) test in comparison. Control growth adequate to meet evaluable assay criteria was obtained in 36 of the 50 tumors tested in the SRC assay (72.0%). In the SDI test, 46 of 50 tumors were evaluable (92.0%). Correlations between the two test systems were dependent upon the activity criteria established for each system. With activity criteria set at a change of less than or equal to -2.0 in the drug sensitivity score for the SRC assay and greater than or equal to 50.0% inhibition of succinate dehydrogenase activity for the SDI test, 12.5% of the drugs tested were active in the SRC assay and 22.3% were active in the SDI test. Correlations of tumor response between the two test systems were 31.7% for sensitivity (13/41) and 95.1% for resistance (98/103). In spite of the fundamental difference between the SRC assay and SDI test, meaningful correlations between the test results and clinical tumor responses in both test systems were obtained. This fact suggests that the two methods are complementary to each other.