Evidence is presented to support the contention that IgM demonstrable by surface immunofluorescent staining on H-2-activated T cells represents specifically adsorbed B-cell-derived alloantibody. T cells activated to H-2 determinants expressed surface IgM only when the progenitor cell populations contained B lymphocytes. IgM was not detected on T cells activated to determinants which fail to stimulate alloantibody production (e.g., M-locus determinants). In addition, IgM-negative H-2 activated T cells (derived from B-cell-depleted lymphoid cells), unlike M-locus-activated T cells, adsorbed IgM in a specific manner when incubated in vitro with "early bleed" antisera raised against the activating H-2 determinants.

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