The presence of surface immunoglobulin (Ig) on Rana catesbeiana lymphocytes was evaluated by immunofluorescence. With all antisera to frog low molecular weight (LMW) Ig and with certain antisera to high molecular weight (HMW) Ig, two populations of lymphocytes, one with and one without surface Ig, were identified. Both populations were present in all lymphoid organs except for the thymus, which had less than 5% Ig-bearing cells. Nonlymphoid cells were not stained. This distribution suggests that these populations are homologous to mammalian and avian B and T lymphocytes. Most cells with surface Ig bear HMW Ig, but a minority bear LMW Ig, and of these a large fraction is distinct from typical lymphocytes morphologically. Unexpectedly, other antisera to HMW Ig, but no control antisera, stained thymocytes, all lymphocytes, and also all other blood cells. This reaction was abolished by absorption with several preparations of HMW Ig purified in different ways. Absorption with frog erythrocytes also abolished the staining of thymocytes and nonlymphoid blood cells, but some lymphocytes continued to be brightly stained; absorption with a non-Ig component of frog plasma had the same effect. Evidently, similar determinants are present on HMW Ig, on a non-Ig plasma component, and on thymocytes and all blood cells. The presence of antibodies to such cross-reactive determinants in certain anti-Ig antisera may lead to the mistaken identification of Ig on cell surfaces.

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