Antigen charge is an important factor in the pathogenesis of experimental immune complex glomerulonephritis. Its potential role in man was investigated in post-streptococcal glomerulonephritis, a disease where the causative agent is known. Cationic, extracellular streptococcal antigens were detected in 8 of 18 renal biopsies from patients with acute poststreptococcal glomerulonephritis (APSGN). The antigen was found mainly in earlier biopsies in which both IgG and IgM were present. Patients' sera taken at the time of biopsy contained antibody to cationic, streptococcal antigens. Cationic moieties are known to have affinity for the glomerular basement membrane and it is possible that the type of antigen described here initiates APSGN via in situ immune complex formation.