When normal human peripheral blood lymphocytes (PBL) were incubated at 37 degrees with soluble transferrin anti-transferrin (TAT) complexes a significant reduction in the proportion of PBL bearing receptors for the reacted Fc portion of IgG(Fc gamma R) was found. Following incubation of such complex-treated PBL in normal human serum the proportion of Fc gamma R + PBL, as assessed by rosette formation with chicken erythrocytes (E) presensitized with rabbit antibody (A) was found to be significantly increased. Such serum-mediated recovery of Fc gamma R was not affected by pretreating PBL with cycloheximide. Recovery was found to be species restricted, Ca++ dependent and confined to a serum fraction containing molecules of relatively low molecular weight (less than 90,000 Mr). Following absorption with EA the restorative capacity of human serum was lost. These findings suggest that following modulation of human PBL-Fc gamma R by immune complexes, receptors may be restored to the cell surface from a serum pool of 'fluid-phase' Fc gamma R. The origin and biological significance of serum Fc gamma R is not known but it is conceivable that they play an important role in immunoregulation.