Serum CA 125, quantified by an immunoradiometric assay employing the monoclonal antibody 0C125 was found to be elevated in 48/58 (83%) of patients with established ovarian cancer. All histological types of carcinoma were antigen positive and there was a positive correlation between the frequency and level of serum CA125 and body burden of tumour. Twenty patients undergoing chemotherapy had serial CA125 estimations following a prospective protocol. Variation in CA125 level reflected disease progression or regression in 21/23 instances. Three of 9 patients tested showed an acute elevation of CA125 in the first week following chemotherapy and this effect predicted a good response to treatment. The natural half-life of CA125 in serum was estimated at approximately 4.8 days, sufficiently short to allow changes in tumour volume to be rapidly reflected by a change in circulating antigen level. Although none of 15 patients with non-Hodgkin lymphoma demonstrated antigen levels outside the normal range, 11/27 patients with non-ovarian adenocarcinoma showed elevated CA125 levels, a specificity of 58% for this latter group. The value of CA125 in the management of ovarian malignancy is discussed.