1 54 of 255 individual human renal biopsies studied by immunofluorescence contained varying combinations of immunoglobulins (Ig), complement (C) components C1q, C3, C4, C5, C6, C8, C3 proactivator (C3PA), and/or properdin. 10 patients had linear deposits of Ig in glomeruli characteristic of antiglomerular basement membrane (GBM) antibodies; nine patients had C3 deposits (minimal in three) with generally lesser amounts of C1q, C4, C5, C6, and/or C8. 118 of the patients had granular deposits of Ig, suggesting immune complex glomerulonephritis; 114 of these had deposits of C3, usually accompanied by C1q, C4, C5, and/or C6. These observations indicate that the entire C sequence is deposited in glomeruli in most Ig-mediated glomerulonephritides. However, certain cases of anti-GBM glomerulonephritis with few or no C deposits may utilize pathways of injury independent of C.21 patients had granular C3 deposits without detectable Ig. C5, C6, and C8 were present in the majority of these patients while C1q was absent and scant C4 was observed in only two patients. The presence of only late-acting C components in the absence of Ig, C1q, and C4 suggests selective, possible nonimmune activation of the alternate C pathway. Finally, five patients had granular deposits of C3, C5, C6, and/or C8 diffusely in all or most glomeruli with a lesser number of glomeruli having additional focal granular deposits of Ig, C1q, and C4. This observation suggests that at least two patterns of C activation can occur simulatenously, possibly triggered by antecedent immune complex deposition and then perpetuated by an as yet undetermined mechanism.