Monoclonal antibodies (H317 and H17E2) against placental-type alkaline phosphatase and testicular placental-like alkaline phosphatase were radiolabelled with 123I or 131I and used in a prospective study of patients with germ-cell neoplasms of the testis (13 studies) or epithelial-origin neoplasms of the ovary (13 studies). The presence of tumour was confirmed in the majority of patients with active disease by antibody scanning. The absence of tumour was confirmed in all cases in patients in complete remission. In two patients with positive antibody scans but no disease found by conventional radiological evaluation, further investigations subsequently revealed the presence of tumour. A positive antibody scan indicates the definite presence of a tumour, although a negative antibody scan does not always exclude the presence of disease. This method may be of clinical value in establishing disease status in patients with potentially curable diseases such as ovarian or testicular cancers.