The binding properties of an IgG1 human myeloma protein, normal IgG, and the Fc and Fab fragments of each were compared in cultures of murine spleen cells. Both 125I-labelled IgG and Fc fragments bound to splenic lymphocytes, whereas Fab fragments did not bind significantly at the highest concentrations tested. On a molar basis, more Fc bound than intact IgG. According to Scatchard plot analysis, the affinity constand of IgG1 was 1.5 x 10(6) +/- 1 x 10(5) L/M and that of the Fc fragments was 7.8 x 10(5) +/- 2.6 x 10(5) L/M. Approximately 25,000 binding sites/cell were calculated for IgG1 and 102,000 for Fc. Deaggregation of the Fc preparation did not change these values, suggesting that the difference in binding of IgG and Fc did not result from Fc aggregation. Unlabelled IgG inhibited about 25% of the labelled Fc binding, whereas unlabelled Fc inhibited approximately 80% of the labelled Fc binding. IgG antigen-antibody complexes, however, inhibited 75% of the Fc binding. In the reciprocal experiment both intact IgG and Fc inhibited the binding of labelled IgG by 100%. The major cell population that bound IgG and Fc fragments in the spleen cell preparation were the B lymphocytes. Removal of macrophages did not significantly affect the binding of labelled Fc fragments. In addition, T-cell-enriched populations bound an insignificant quantity of Fc fragments.