Abstract Human sera containing complexes of lactatedehydrogenase (LDH) and immunoglobulin A (IgA) were studied. Such complexes could be demonstrated in a number of sera with abnormalities in electrophoretic pattern of the LDH isoenzymes. LDH-IgA complexes were observed in a number of sera in which the LDH-2 band was missing, in a serum with all LDH activity in the β-γ globulin region, but also in some sera with only minor abnormalities in the LDH isoenzyme pattern. It is shown that the IgA bound to LDH is of the kappa light chain type. Auto-antibodies were not detected in the sera. Therefore, the formation of the LDH-IgA complexes does not seem to be the result of an antigen-antibody reaction.