Samples of 38 human renal cell carcinomas (RCC) were subjected to routine histopathological examination but also to in vitro sensitivity testing with mitomycin C, vinblastine and interferon Alpha-2a at various concentrations corresponding to serum titers recommended to be effective in vivo, employing a monolayer assay. Extending earlier in vitro studies, both tumor cell kill rates (TCKR) and proliferation rates (PR) were assessed. Following in vitro preparation the tumor cell cultures were simultaneously exposed to the anticancer drugs listed above. The proliferation rates were determined immunocytochemically using the monoclonal antibody Ki-67. Nine (23.7%) of the tumors investigated revealed temporary and limited response with respect to either TCKR or PR. Improvement of this percentage could only be obtained by increasing drug concentration to titers with toxicity intolerable for in vivo administration. The in vivo data presented correspond to clinical temporary and limited remissions in patients with metastatic RCC ranging up to 25%.