Sera from four latex-allergic hospital employees (two physicians and two nurses) were examined for the occurrence of IgG4 and IgE antibodies to rubber proteins by immunoblotting. We used natural rubber sap as antigen source and detected IgG4 antibodies in all four patient sera. The IgG4 antibodies bound to 12 rubber protein antigens with molecular weights ranging from 14 kD to 53 kD. IgE antibodies bound nine of the antigens and the major antigen for both IgG4 and IgE seemed to be a 21 kD rubber protein. The parallel occurrence of IgG4 and IgE antibodies against the same rubber protein antigens suggests that IgG4 antibodies also may play a role in the pathogenesis of latex allergy.