One hundred surgical specimens were used for the clonogenic assay to determine the chemosensitivity in vitro and the results were compared with the antitumor effect of the adjuvant cancer chemotherapy after operation. The clonogenic assay was performed according to the method of Salmon et al. The dissociated tumor cells were contacted with drugs in a concentration of 1 microgram/ml for 1 hr. (mitomycin C; MMC) or 2-3 wks. (5-fluorouracil; 5-FU). Fifty percent or more inhibitory rate of the colonies were evaluated as positive. The clinical antitumor effects were assessed by the tumor reduction in non-curatively operated cases and by the survival period after the operation in curatively operated cases. Overall evaluable cases were 52 of 100 cases with a median cloning efficiency rate of 0.1%. The efficacy rates of the drugs in vitro were found to be 21.3% for MMC and 31.8% for 5-FU. The histological differentiations, Stages, pretreatments and cloning efficiency rates were observed to have few influence on the chemosensitivity. Nineteen non-curatively operated cases were eligible for the comparison between in vitro and in vivo, showing 40% true positive, 100% true negative and 84.2% predictable rates. In curatively operated cases, whereas 2 of 14 cases have died of cancer in not adapted adjuvant cancer chemotherapy group, no cancer deaths were encountered in 7 cases whose adjuvant cancer chemotherapy was evaluated as effective in the clonogenic assay. From these findings, it was concluded that in vitro clonogenic assay might be useful to select a suitable adjuvant cancer chemotherapy in curatively operated cases as well as cases with advanced carcinomas.