A mother developed hematuria during the fourth month of pregnancy, and her nursing infant son from this otherwise uncomplicated pregnancy developed hematuria at 3.5 months of age. Both had a mild glomerulonephritis characterized by mesangial prominence, focal thickening and mottling of the glomerular basement membrane and electron-dense deposits, predominantly in the intramembranous and subendothelial positions. Immunofluorescence studies revealed striking accumulations of C3 and other complement components associated with alternative complement pathway activation within glomeruli, and the presence of small or equivocal amounts of immunoglobulin. C1q, C4 and factor B were not detectable. The glomerular lesion was accompanied by hypocomplementemia. Sera of both mother and infant displayed half normal levels of C3 and factor B, increased levels of C4, and normal levels of 12 other complement proteins. High normal or slight elevation in nephritic factor-like activity was observed in serial serum samples. Studies suggested that this mother and son represent the second kindred having an abnormal form of C3 which produces an alternative complement pathway C3 convertase, C3b, Bb, resistant to control by factor H. No additional affected family members were identified. The course of the nephritis over 7 years without drug therapy has been mild with resolving hematuria and no abnormal proteinuria or decrease in creatinine clearance.