Direct immunofluorescence, immunoelectron microscopy, and special immunohistochemical procedures including guinea pig complement fixation, differential elution, and in situ antigen binding were employed in an immunomorphologic analysis of kidney biopsy specimens from 98 children with clinically diagnosed nephrotic syndrome and/or glomerulonephritis (GN). Glomerular deposits of hepatitis B virus (HBV) antigens, immunoglobulins, and complement were detected in specimens from 24 children, all seropositive for hepatitis B surface antigen (HBsAg) and antibody to hepatitis B core antigen (HBcAg). Of these, 21 cases were diagnosed as membranous glomerulopathy (MGN), 1 as membranoproliferative GN, and 2 as diffuse mesangial proliferative GN. HBaAg was identified as the only HBV antigen in about a third of the cases of MGN, whereas in another third it was accompanied by HBsAg. HBsAg was the only HBV antigenic component detected in the glomerular deposits in the remaining third of the cases of this GN form. The results of this study indicate that apart from, or in addition to, HBsAg immune complexes, HBcAg immune complexes may also participate in the pathogenesis of a significant number of MGN cases in children subclinically infected with HBV. A possibility that these complexes include nonparticulate, presumably low-molecular-weight HBaAg components and that they are found in an environment of antibody-excess is discussed.