The defined, purified ovarian cancer-associated antigen, NB/70K, was used as tumor antigen in the humoral leukocyte adherence inhibition (H-LAI) assay. Bell-shaped dose-response relationships were obtained with increasing concentrations of NB/70K and constant serum concentration from ovarian cancer patients. The concentrations of NB/70K used in this study were 100-250 times less than what had been used in previous H-LAI studies with crude antigen extracts. The response rate of ovarian cancer patients was greatly influenced by the stage of the disease. Thus 75% (6/8) of the patients with disease restricted to the pelvis (stages I and II) reacted while only 20% (5/24) of the patients with more advanced stages (stages III and IV) gave response. Of patients with other gynecological cancers, 17% (1/6) were positive and only 8% (1/13) of patients with other malignant diseases reacted. One of the 11 controls (9%) was found to give positive response. The importance of the present investigation was the finding that a defined, purified, cancer-associated antigen elicited reactivity in the H-LAI assay and that a high sensitivity for detection of early stages of ovarian cancer was obtained with this antigen. The results suggest that TAAs involved in immune recognition of cancer can be identified by the H-LAI assay.