Optical colony counting was used for assessing drug sensitivity of 87 samples (from 51 primary solid tumors, 19 metastatic solid tumors, and 17 ascites fluids) from 83 common epithelial type ovarian carcinomas. The evaluability rate by the optical counting method was 26.4%. Fifty of these samples were also assessed by a tritiated thymidine ( [3H]dThd) incorporation assay (evaluability rate by incorporation assay, 74%). In the 12 cases fully evaluable by both methods, the correlation coefficients for regression of drug sensitivity data between the methods ranged from 0.503 to 0.976 (mean, 0.745); for the combined data of these cases, the correlation coefficient was 0.707. The new [3H]dThd assay appears useful and efficient for performing in vitro chemotherapy sensitivity testing of human ovarian cancer.