Cryostat sections of human kidney were incubated with C3b containing immune complexes. The complexes were horseradish peroxidase (HRP) and F(ab')2 fragments of rabbit IgG anti-HRP incubated in C4-deficient guinea pig serum. The peroxidase activity was revealed histochemically. The sections were studied using light and electron microscopy. The bound complexes were localized to the foot processes of the visceral epithelial cells and to the surface of the parietal epithelial cells facing Bowman's space. No receptors could be demonstrated on the endothelial or mesangial cells.