The rabbit knee joint was used to evaluate the inflammatory properties of immune complexes with different contents of antigen. Immune complexes made in vitro with antiserum from hyperimmunized rabbits and bovine serum albumin as antigen were injected into the joints. The reactivity of the individual animal was established by injection of normal rabbit serum into the contralateral joint. The number of leukocytes washed out from the joints at fixed intervals was used as a measure of the inflammatory properties of the immune complexes. With immune complexes formed at antigen excess the highest numbers of leukocytes, with a predominance of granulocytes, were found after 6 h of exposure. However, these complexes gave only weak complement activation in vitro, measured with a hemolysis assay. Complexes formed at optimal precipitation proportions gave high complement activation in vitro but only a small number of leukocytes in the joints at 6 h exposure. This finding may be explained by a rapid phagocytosis of the latter complexes while complexes formed at antigen excess are not readily phagocytized. This hypothesis is supported by the observation that higher leukocyte counts were found after shorter exposure times to complexes formed at optimal precipitation proportions than after exposure to complexes with antigen excess.