[en] Use of the Hamburger-Salmon soft agar assay method for in vitro chemotherapy sensitivity testing of samples of renal cell carcinoma has been somewhat limited by a relatively low proliferation/evaluability rate for this tumor type (approximately 50%). The tritiated thymidine ([³H]-TdR) incorporation assay method of Tanigawa et al. (Cancer Res., 42: 2159, 1982) was compared to a standard optical colony counting assay technique. Fifty-seven different primary and five metastatic fresh samples of human renal cell carcinoma were studied. Evaluability rate by the [³H]-TdR assay was 90% (greater than or equal to 300 cpm control). In comparison, evaluability rate by optical colony counting was 43% for this group of tumors. [³H]-TdR incorporation increased with increasing tumor grade and increasing stage. Spindle cell tumors showed significantly higher cpm than other cell types. Twenty-three primary tumors were evaluable by both [³H]-TdR and colony counting methods. The correlation coefficient (r) for regression lines for drug sensitivity data points (optical counting vs. [³H]-TdR) of these individual experiments ranged from 0.50 to 0.99 with a mean r +/- S.D. of 0.76 +/- 0.15. For all 260 paired drug response observations of 23 tumors exposed to different drugs, the correlation was very good with r = 0.71. Since the [³H]-TdR assay has an evaluability rate of approximately 90% for renal cell carcinoma, gives drug sensitivity information which correlates well with the colony counting endpoint and yields chemotherapy sensitivity information four days after sample accession, the [³H]-TdR assay may be a more useful method for study of human renal cell carcinoma in vitro chemotherapy sensitivity testing than standard colony counting techniques