Antibodies, found in human sera from patients with poststreptococcal glomerulonephritis, against proteoglycans (PG) derived from bovine and human glomeruli were investigated. PG were isolated by 4 M guanidine-HCl extraction of whole glomeruli, followed by DEAE-Sepharose CL-6B ion exchange chromatography. The anionic fractions were further purified by chromatography on Sepharose CL-4B. Biochemical analysis of the two resulting peaks revealed the presence of high molecular weight anionic material containing protein, uronic acid, glucosamine, and galactosamine. Enzymatic and chemical susceptibilities indicated the presence of heparan sulfate PG and a galactosamine-containing PG. Immunologic studies revealed the presence of anti-PG antibodies to both PG peaks of the Sepharose CL-4B column in glomerulonephritis sera. Inhibition studies using an ELISA demonstrated that heparan sulfate was a major antigenic determinant. Cross-reactivity with both mammalian and streptococcal hyaluronate was noted. Inhibition studies also indicated the presence of a second antigenic site containing N-acetylgalactosamine, possibly representing chondroitin or dermatan sulfate PG.