The frequency of hepatitis B surface antigen (HBsAg) has been studied in the sera and renal biopsies of 276 patients with various forms of glomerulonephritis (GN), the nephrotic syndrome and other nephropathies. Using a modified Hepanosticon method, HBs antigenemia was detected in 32 of 196 patients (16.3%) with immune complex (IC) GN and the nephrotic syndrome. Indirect immunofluorescence revealed HBsAg in 33 renal biopsy tissue specimens (16.8%). HBsAg was found in the sera of four of the 80 remaining patients with other renal diseases (5%), and in the renal biopsy tissues of another four (5%). Antibody against HBsAg could only be demonstrated in the serum of one glomerulonephritic patient. The sera of 18,799 normal blood donors were used as controls; of these 186 (0.99%) had positive tests for HBsAg. It is concluded that, in some patients with GN and the nephrotic syndrome, HBsAg-containing IC may be implicated in the development and/or progression of the disease.