A patient with asthma urticaria and angioedema induced by allergy to seminal plasma was examined at intervals for 10 years. Before treatment her anaphylactic susceptibility to seminal plasma was manifested by very strong prick-test responses, IgE antibody to an allergenic fraction of seminal plasma determined by RAST, and by antigen-induced histamine release from her blood leucocytes. The skin test and in vitro lymphocyte tests indicated concomitant delayed hypersensitivity to the same allergen. The patient's lymphocytes treated with seminal plasma allergen fraction showed much increased incorporation of thymidine, and also synthesis of a product (NIF) that inhibited migration of neutrophils from a normal donor. The allergen fraction of seminal plasma had about five components in the range of 20 000-40 000 daltons molecular weight; the major fraction binding IgE appeared to be a glycoprotein. The patient was successfully desensitized by injections of her husband's seminal plasma. Desensitization was not associated with persistent amounts of antigen-specific IgG antibodies.