Second generation CA 125 assays are expected to replace the original CA 125 assays soon. This study compares the Centcor CA 125 II and the original Centcor CA 125 assays in 50 women: 34 with gynecologic cancers, 5 with benign conditions and 11 healthy. The serum values obtained by both assays were not statistically different and showed high correlation especially for values < 300 U/ml. Two patients who had false elevation of CA 125 by the original assay secondary to human antimouse antibody had normal values by the CA 125 II assay. It is concluded that CA 125 serum values obtained by the Centcor CA 125 II assay correlate well with the values obtained by the original Centcor CA 125 assay especially for values < 300 U/ml and are less likely to show elevated false positive results secondary to human antimouse antibody.