The response of gastric cancer to intravenous application of low-dose CDDP plus 5-fluorouracil (FP) was assessed by changes in DNA indices (DIs) and PCNA labeling indices (LIs) by flow cytometry, and by the thymidylate synthetase inhibition rate (TSIR). Pretherapy samples were compared to specimens resected at operation. FP therapy was performed in 10 of 26 specimens and not in the others (C). There was no difference between change in DIs of FP group and that of C group. In FP group, PCNA LI of resected specimen was higher than that of biopsy, although they showed almost the same values in C group. Change in PCNA LIs correlated to TSIR in FP group. These results suggest that FP therapy affects gastric cancer cells in PCNA LIs and TSIR, and that PCNA LIs could be an indicator of the effect of FP therapy as TSIR.