Antibodies to hormones have been used extensively to study endocrine regulation of reproduction and in recent times they have been commercially exploited as therapeutic agents to manipulate reproduction. Despite the release of two commercial products, little is known about the mechanism of action of immunoneutralization. The presence of hormone-binding antibodies in the circulation and the detection of hormone antibody complexes in blood has led investigators to suggest a vascular site of action for immunoneutralization. However, a vascular site of action is difficult to reconcile with some of the known effects of immunoneutralization to the boar taint steroid (5 alpha-androst-16-en-3-one) to androstenedione and to gonadotrophin-releasing hormone. In this paper we review some of our recent research into the mechanism of action of androstenedione immunoneutralization. We suggest that hormonal immunoneutralization of androstenedione may take place in the extracellular compartment or within the plasma membrane where antibodies to androstenedione may bind to a specific steroid-binding protein in the plasma membrane of granulosa cells.