An evaluation was made, using in vivo subrenal capsule (SRC) assay, of the chemosensitivity of 12 human neoplastic tissues including 7 gastric cancers, 2 colo-rectal cancers, 2 lung cancers and 1 breast cancer, exposed to 5-fluorouracil (5-FU) and the 5-FU derivatives: tegafur (FT), UFT and hexylcarbamoyl-5-FU (HCFU). The relative variation of tumor size (delta TS/TS0) was calculated as follows: delta TS/TS0 = (TS6-TS0)/TS0 X 100 (%), where TS6 was the tumor size on day 6 and TS0 that on day 0, and more than a 10% decrease of delta TS/TS0 in the treated group was considered as positive for chemosensitivity. Delta TS/TS0 was decreased to -10.9 +/- 11.3% (mean +/- standard deviation) for 5-FU, -13.0 +/- 16.4% for FT, -19.8 +/- 18.2% for UFT and -18.8 +/- 15.1% for HCFU, and the inhibition of tumor growth following exposure to 5-FU correlated well with that following exposure to 5-FU derivatives (5-FU vs. FT, r = 0.659; 5-FU vs. UFT, r = 0.864; 5-FU vs. HCFU, r = 0.729). However, the sensitivity of each tumor varied; for example, the chemosensitivity was positive for 5-FU derivatives, but negative for 5-FU, in some tissues. The SRC assay appears to be useful for selecting sensitive drugs from 5-FU and 5-FU derivatives for individual cancer patients.