Following an attack of infectious mononucleosis, the red cells of HLA B7 patients may show a greatly increased reactivity with anti-Bg^a antibodies. This occurs from about the 3rd week of the disease, the reactivity then slowly decreasing over a period of months or years. Both specific and nonspecific reactions to other HLA antisera may also occur. Five patients were followed in detail and the reason for the increase in antigen strength was investigated without a conclusive result.