Rheumatic disease sera were examined by a sensitive and specific enzyme linked immunosorbent assay (ELISA) for antibodies to native and to denatured type IV collagen from basement membranes of bovine anterior lens capsules or human placenta. The frequency of antitype IV collagen antibodies depended on the antigen and the disease studied. No controls had human type IV collagen antibodies and only 5.6% had antibody to bovine type IV collagen. Antibody to one or more of our 4 antigens was seen in 20% of children with juvenile rheumatoid arthritis (JRA), 35% of patients with mixed connective tissue disease (MCTD), 40% of those with juvenile dermatomyositis (DM), 52% of adults with rheumatoid arthritis (RA), 56% of patients with scleroderma and 60% of patients with systemic lupus erythematosus (SLE). Antibodies to native human type IV collagen were rare (0-10%) except in SLE (45%). Antibodies to denatured human type IV collagen were commoner in RA, scleroderma and SLE. Antibodies to native bovine type IV collagen occurred in 8-20% of patient sera, and to denatured antigen in 25-26% of scleroderma, MCTD and DM patients. Antibovine type IV collagen activity measured by ELISA could be absorbed from positive sera by preincubation of the sera with bovine type IV collagen but not bovine type I collagen or native human placental type IV collagen, indicating that the antibodies were specific for bovine type IV collagen.