A quantitative immunoblotting assay (QIBA) for the determination of specific antibody titres in human autoimmune sera is described. In this assay, a total HeLa nuclear protein fraction, immobilized on nitrocellulose blot strips, was used as source of antigens and immunoreactive species of autoantibodies were quantitated by an enzyme linked second antibody procedure. Besides being more discriminative, QIBA appeared to be up to 500 times more sensitive than immunodiffusion or immunoelectrophoresis. In this study we used 21 sera from patients with SLE or MCTD for a quantitative analysis of their specific autoantibody content. Within this group, a very diverse spectrum of antibody populations was observed; anti-RNP sera appeared to contain, among others, high tired antibody versus 70K and 31K polypeptides while all (n = 6) anti-Sm sera recognized a 25kD protein doublet. In a follow-up study of two MCTD patients significant flares in specific antibody content could be observed.