Proteinase K-treatment of red blood cells either diminished or abolished the antigenic activities of glycophorin A and glycophorin B, and revealed the presence of a cryptic antigen that was recognized by antibody naturally existing in the autoplasma. About ninety five percent of all healthy persons have this autoantibody belonging to the IgM classification, whose titer ranges from 2 to 32. The activity of this autoantibody was absorbed by histidine and glutaminic acid. We were able to isolate this autoantibody from the plasma by means of an alkaline elution method and the autoantibody did not agglutinate chymotrypsin-treated red blood cells and red blood cells treated with chymotrypsin, following proteinase K-treatment. These results indicate that after proteinase K-treatment this autoantibody may not have an affinity for glycolipids, but for proteins digested by chymotrypsin.