A new combined enzyme linked immunoassay (ELISA) was developed to measure both serum placental-like alkaline phosphatase (PLAP) activity (PLAPA) and concentration (PLAPC) in the same microtitre plate using an Imperial Cancer Research Fund monoclonal antibody, designated H17E2. PLAP A and PLAP C were determined together with an existing marker, CA125 in 397 serial samples from 87 patients with epithelial ovarian cancer. Retrospective assessment showed the sensitivity to increase from 73% with CA125 alone, to 88% using CA125 and PLAP A, and to 93% with all three markers in 261 samples from the patients with known active disease at the time of sampling. When the results for all 397 samples were included in the analysis, however, the specificity, sensitivity, accuracy and predictive powers of this monoclonal antibody were not sufficiently high to assist in the prospective follow up of patients with ovarian cancer. This was due to a significant number of false positive and false negative results. Our data indicate that PLAP A or PLAP C estimation with H17E2 may, therefore, only be of value in the management of those patients with known active disease who are already known to be "marker positive" for this antigen.