Macrophage receptor sites for IgG are important in the immune clearance of particles both from the blood and lung. We studied the number, affinity, and density of binding sites for monomeric IgG on human blood monocytes and alveolar macrophages. Monocytes and alveolar macrophages had a similar affinity for monomeric IgG at 37 degrees and 4 degrees C. The half-time for dissociation of the IgG-receptor complex was also similar for both cells. However, alveolar macrophages expressed approximately 5-fold more IgG binding sites than monocytes at both 37 degrees and 4 degrees C. Nevertheless, when cell surface area was estimated, these cells expressed a similar density of IgG binding sites (monocytes = 110 +/- 14.8 IgG binding sites/square micron; pulmonary macrophages = 138 +/- 46.9 IgG binding sites/square micron; p greater than 0.50). Gamma interferon increased the number and density of monocyte binding sites for monomeric IgG by 162 +/- 89%. Furthermore, patients with sarcoidosis, a disorder in which gamma interferon is spontaneously elaborated, expressed a similar increase in the number of IgG binding sites per monocyte, from 24,968 +/- 1,361 for normal subjects to 44,860 +/- 6,652 for patients with sarcoidosis. However, alveolar macrophages from 5 patients with sarcoidosis expressed a normal number of IgG binding sites. These data suggest that there is no major alteration in the Fc(IgG) receptor as monocytes differentiate into alveolar macrophages. Both gamma interferon treatment and sarcoidosis are associated with enhanced expression of the Fc(IgG) receptor on monocytes.