A monoclonal antibody (NDOG2) against placental alkaline phosphatase (PLAP) in ovarian cancer has been used in three ways by the Bristol University Department of Obstetrics & Gynaecology. First, in an indirect immunoperoxidase technique, NDOG2 demonstrated positive standing in 64% of 56 ovarian carcinomas as well as in 25% of 44 benign tumours. The majority of these positive tumours were serous cystadenocarcinomas or serous cystadenomas and there was considerable variation in the expression of this antigen from tumour to tumour. NDOG2 was also used as the basis of two serum assays and, when labelled with 123-iodine (123I), in radioimmunoscintigraphy (RIS) to monitor patients' response to therapy. The first serum assay measures the enzymic activity of PLAP and the second recognizes the antigenicity of the molecules. Assay 2 proved more useful in that it predicted the course of the disease in 45% of patients followed up, whereas Assay 1 was only of use in 25% of cases. RIS proved to be a useful imaging technique and was at least as sensitive as conventional imaging techniques. The common causes of false-positive and false-negative results are described.