Localization of fluorescein-conjugated heat-aggregated IgG (FA IgG) was demonstrated by immunofluorescence in renal glomeruli of 16 of 69 patients with glomerulonephritis. FA IgG bound more frequently in kidney biopsies from patients with diffuse glomerulonephritis and depressed renal function, and localized selectively in glomeruli that contained heavy deposits of IgM, C3, and C4. The factors that caused FA IgG to bind were specifically reactive with the Fc piece of the IgG molecule and were resistant to 56 degrees C heat for 30 minutes. Localization of FA IgG in the kidney did not correlate with the presence of soluble immune complexes or detectable antiglobulin antibodies in the sera. Binding of FA IgG was also seen in glomeruli and arteries of 18 of 21 kidney allografts studied at the time of impending rejection. But the factors responsible for binding FA IgG in the allografts were heat labile and thus could have been C1q. Although the role of these "antiglobulins" in the immunobiology of glomerulonephritis remains unknown, the fact that they occurred mainly in patients with relatively severe glomerular injury suggests that they could play some part in promoting renal glomerular injury.