Small and medium lymphocytes from the peripheral blood and lymphoid tissues of the rabbit react in suspension with antibodies directed against different immunoglobulin determinants. Through immunofluorescence, it was possible to show that numerous discrete spots on the surface of the positive lymphocytes carry immunoglobulin molecules.

The positive lymphocytes are about one-half of all lymphocytes in the different preparations; thymus lymphocytes are all negative.

With antisera specific for rabbit IgM as well as with antisera directed against allotypic determinants specific for IgM or IgG, it was possible to show that about nine-tenths of the immunoglobulin-positive lymphocytes carry IgM molecules on their surface.

With antisera directed against a- and b-locus determinants, it was also possible to demonstrate that both heavy and light chains were present in the surface immunoglobulins. Furthermore, in animals which were heterozygous at the a or the b locus, it was found that each lymphocyte had immunoglobulins synthesized under the influence of only one of two alleles.

A very small proportion of lymphocytes could be shown to have a specific surface reaction with one antigen (horse ferritin); the proportion of these cells increased very much after immunization.

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