Using a murine acute serum sickness model caused by cationized bovine γ-globulin (CBGG), the histological findings, accumulation of CBGG in the organs and the effect of decomplementation for the clearance of immune complex (IC) were investigated. The accumulation of CBGG increased in the lungs in the presence of anti-CBGG antibody 1 hour after injection of CBGG, but did not change in the kidneys . This suggests that CBGG forms IC in situ in the kidneys, and that circulating IC accumulates in the lungs. Decomplementation did not influence the uptake of CBGG immediately after challenge but delayed the clearance of CBGG in the kidneys, lungs, liver and spleen. A beneficial role of the complement system in the clearance of IC even for those formed in situ was reco gnized. Histological changes, cellular infiltration and microvascular destruction, evoked in the lungs in this experimental model immediately after challenge, were not seen in the kidney, suggesting the different modes of complement activation in these organs.