IgG subclass distribution was determined in glomerular immune deposits found in patients with membranous, mesangiocapillary, lupus and antiglomerular basement membrane antibody induced glomerulonephritis. In each disease category IgG3 was the predominant subclass found. In membranous, lupus and anti-glomerular basement membrane antibody induced nephritis the other subclasses were detected in significant but lesser amounts although in anti-glomerular basement membrane antibody induced nephritis IgG2 deposition was minimal. Particularly striking was the excess of IgG3 compared with other subclasses in mesangiocapillary glomerulonephritis and the greater amount of IgG4 in membranous glomeruli compared to the other disease categories. These findings indicate a difference between the distribution of IgG subclasses in normal plasma and glomerular immune deposits and may be of importance in the pathogenesis of the types of glomerulonephritis studied.