An exceptionally rare case of spontaneous allergic sensitization to a glycoprotein rich in sialic acid and present in human seminal fluid is described in a woman. An anaphylactic reaction, consisting of giant urticaria, oedema, asthma, uterine pain and cardiovascular collapse followed within minutes of coitus. A high degree of atopic hypersensitivity was shown by skin reactions to human seminal fluid, but not to that of animal origin. Passive transfer of the hypersensitivity to man and monkey was successful and the responsible antibody was shown to be reagin-like and associated with the group of γA-globulins.