Plasma levels of the anaphylatoxins C3a and C4a were examined in patients suffering with various renal diseases. Raised levels were observed in a considerable number of patients; although such elevation could be ascribed to in vivo complement activation in cases of immune complex glomerulonephritis, we found that raised plasma C4a levels appeared to be related to impaired renal function, suggesting that C4a anaphylatoxin is cleared by the kidneys. No such relationship was found in the case of the anaphylatoxin C3a suggesting the possible existence of another mechanism of elimination of C3a.