Using antibody coated bovine erythrocytes we were unable to demonstrate Fc-receptors on either thymus cells or T cells prepared from lymph node cell suspensions by anti-Ig column filtration. However, if parental thymus or lymph node T cells were transferred to X-irradiated F1 hybrids, activated donor T cells recovered from the recipient's spleen (ATC-spleen) were shown to express Fc-receptors. Fc-receptors were also demonstrable on ATC-spleen prepared between strain combinations differing at the M-locus. In marked contrast, Fc-receptors were not detected on ATC recovered from thoracic duct lymph (T.TDL). This applied to (a) H-2-activated T.TDL derived from normal thymus cells, (b) H-2-activated T.TDL derived from thymus cells depleted of B cells, and (c) M-locus-activated T.TDL. Of these three populations, surface Ig (of B cell origin) was detected on a large proportion of the first but not on the second and third populations. Thus, the failure to detect Fc-receptors on any of these populations could not be attributed to blocking by adsorbed surface Ig. In addition, various T-cell populations were examined by a microcytotoxicity assay for the presence of cell surface Ia-antigens. 5--10% of the thymus cells, 20--30% of cortisone-resistant thymus cells, 60--70% of lymph node cells, and 60--80% of ATC-spleen and T.TDL showed Ia.

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