In tuberculin-sensitive individuals, IgG Fc receptor (FcR)-bearing lymphocytes in the peripheral blood increased transiently following PPD-tuberculin skin test. This rise in circulating FcR-bearing cells appeared to peak about 36--48 h after the intradermal inoculation of PPD and seemed to occur largely in the T cell population. Skin test-negative individuals showed no significant changes in their circulating FcR-bearing cells following PPD inoculation. Peripheral blood lymphocytes from PPD-sensitive individuals were fractionated into non-T cell and T cell-enriched populations by E rosette sedimentation technique. FcR-bearing cells in the T cell-enriched population were eliminated by EA rosette sedimentation: i.e. FcR-negative T cells. Then, equal numbers (1 X 10(5) cells each) of non-T cells and unfractionated or FcR-negative T cells were recombined in culture. Prior to PPD inoculation, there was no significant difference between these two cell mixtures in the in vitro cellular response to PPD or mitogens. When these cell populations were obtained 36--48 h after PPD inoculation, however, the combination of non-T cells and FcR-negative T cells responded to PPD much better than the combination of non-T cells and unfractionated T cells, whereas the mitogen-induced cellular proliferation of these two cell mixtures did not differ from each other.