We investigated whether the proliferating cell nuclear antigen (PCNA) protein takes part in cw-diamminedichloroplatinum(II) (CDDP) resistance, using a murine leukemia cell line P388 and its CDDP resistant cell line. P388/CDDP was 4 times more resistant to CDDP than P388. The cell lines were maintained in the DBA/2 female mouse peritoneal space. In total cells, the amount of the PCNA protein decreased to 90% in P388 after 1 h CDDP treatment, but that of P388/CDDP increased to 127%. The difference was statistically significant (p = 0.012, n = 5). As for G2/M phase cells, the difference was also significant at 1 h (p = 0.016, n = 5) and at 2 h (p = 0.036, n = 5) after treatment. In P388 the amount of the PCNA protein decreased in accordance with the inhibited cell proliferation, whereas in P388/CDDP, the amount of the PCNA protein increased in spite of the inhibited cell proliferation. This increase of the PCNA protein suggests that the PCNA protein is involved in CDDP resistance of P388/CDDP through enhanced DNA repair synthesis.