Lymphoid cells from thymus, thoracic duct lymph (TDL), and thoracic duct lymph in irradiated animals reconstituted with allogeneic thymus cells (TTDL) were labeled with radioiodinated anti-immunoglobulins using radioautographic techniques. Thymus and TTDL were labeled (14.4 and 37.0%, respectively) with anti-light chain protein after prolonged exposures (30–60 days). No labeling was observed on thymus and TTDL with anti-polyheavy chain globulin. In contrast 18.5–19.0% of TDL labeled on short exposure (6 days) with anti-polyheavy chain and anti-light chain materials. It is proposed that the difference between the labeling observed on short exposures versus long exposures can be related to the difference in surface density of immunoglobulins between nonthymus-derived (B) and thymus-derived (T) cells.

The distribution of labeled cells in the thymus was preferentially among the larger cells (greater than 10 µ diameter). The TTDL population was mostly composed of a larger, blast-like population and the distribution of label was independent of size.

As the thymus and TTDL preparations contain almost exclusively T cells, this represents a direct demonstration of surface immunoglobulin light chains on T lymphoid cells.

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