Three patients with a 2-3 year history of parenteral abuse of pentazocine hydrochloride (Talwin) developed membranoproliferative glomerulonephritis type I. Clinical presentation included nephrotic syndrome, microscopic hematuria, and hypertension. Tissue studies disclosed 1+ to 4+ mesangial and subendothelial deposits of immunoglobulins (predominantly IgM) and complement components C1Q, C3 and C4 with focal reduplications of glomerular basement membranes. Glomerular C3 receptor studies performed on one patient demonstrated diffuse loss of receptor activity correlating with localization of immune deposits. Serum studies showed decreased levels of C3, elevated levels of IgM (greater than 2 SD) (three patients) and circulating immune complexes (two patients). Repeated blood and urine cultures were negative. Immune mechanisms may be involved in the pathogenesis of Talwin addict nephropathy.