An autoradiographic binding assay employing 125I-labeled heat-aggregated mouse IgG2b myeloma protein (MOPC 141) was used to demonstrate receptors for IgG on 20–45% of Balb/c thymocytes and on 70–80% of splenocytes. Binding could also be shown with heat or BDB aggregates of another IgG2b (MOPC 195), with IgG1 and with human γ-globulin, but not with aggregated chicken γ-globulin, IgA, BSA, nor with aggregated Fab fragments of IgG2b. Optimum binding was obtained at 37°C. Detection of binding was dependent upon aggregate size with complexes of more than 100 IgG molecules being optimal, aggregates of 6–25 detecting splenocytes but not thymocytes, and aggregates of less than 6 binding to a negligible extent. Comparison of grain counts on various cell types showed mastocytoma cells (P815) and macrophages averaging 40–50 grains/cell/day, allogeneically activated thymocytes 20–30, splenocytes 2–3, L5178 lymphoma cells 1, and positive thymocytes 0.6 grains/cell/day. Double labeling experiments for surface Ig, θ-antigen, and agg IgG receptor on mouse spleen cells indicated that a relatively high density of receptor was present on about 80% of B cells, 30% of T cells, and 60% of SIg-, θ-, null cells.

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