The Fc receptor activity of single macrophages and of distinct macrophage populations was examined in terms of cellular avidity for IgG antibody by using a rosette assay which permits characterization of receptor activity at the cellular level and quantitation of any changes in such receptor activity. Marked heterogeneity was shown to exist within normal populations of macrophages. Both normal peritoneal and alveolar macrophage populations exhibit a normal or logistic distribution of cellular avidities for IgG antibody. The mean avidity of the peritoneal population, however, was found to be approximately 3 times greater than that of the alveolar population. Moreover, alveolar macrophages possess a range of cellular avidities 3 times greater than that of peritoneal macrophages. Comparison of normal unstimulated peritoneal macrophages and induced inflammatory exudate macrophages revealed a 6-fold increase in the proportion of high avidity cells in the latter population. Normal peritoneal and normal alveolar macrophages were shown to undergo a striking increase in Fc receptor activity in vitro. Under certain conditions of culture these cells acquire the antibody-binding capacity characteristic of high avidity cells in the inflammatory exudate population. Fresh guinea pig serum was shown to prevent this activation of Fc receptor function in vitro. Aged guinea pig serum was somewhat less effective. The serum factor(s) responsible appears to be consumed or exhausted in some way by macrophages. It is heat stable and has a molecular weight of less than 100,000 daltons. Serum IgF was found not to be involved in the effects of fresh serum on receptor activation in vitro. The activation of Fc receptor function described here may facilitate both the presentation of antigen to specific T lymphocytes and the antibody dependent killing of tumor cells by macrophages. Changes in receptor activity of the kind described may thus play an important regulatory role in the induction of an immune response and in the effector mechanisms of immunologic surveillance.