In vivo and in vitro chemosensitivity testing has been applied to kidney carcinoma tumor lines. Serially transplantable tumors were implanted subcutaneously in nude mice and the animals were treated with cytostatic drugs. The results of this in vivo assay were compared with the results obtained with the subrenal capsule assay, the DNA precursor assay (3H-thymidine), and the colony-formation assay, utilizing the same tumor line in each case. Higher rates of resistant tumors were found in the in vivo assays than in the in vitro assays. The SRC assay and the DNA assay had the highest predictive value, as judged from comparison with the results obtained with the source tumor (human kidney carcinoma tumor line).