The present studies confirm that small doses of human seminal plasma inhibit mitogen induced lymphocyte transformation and experiments in a murine model suggest this effect may be mediated through accessory macrophages. They also reveal that seminal plasma interferes with the attachment, spreading and phagocytic activity of C. parvum elicited murine peritoneal macrophages and the release of reactive oxygen species from zymosan triggered human peripheral blood monocytes and neutrophils. As macrophages are known to programme various aspects of the immune response our results may be relevant to the acquired immune deficiency syndrome (AIDS), the genesis of antisperm antibodies and the response to prostatic and cervical cancer.