Membrane immunoglobulin is present on thymic and splenic lymphocytes of the trout Salmo gairdneri.

Three rabbit antisera raised to trout IgM showed positive immunofluorescent reactions with lymphocytes of trout spleen and thymus. These antisera showed no detectable cross-reactivity with keyhole limpet hemocyanin, as determined by direct radioimmune precipitation and adsorption, and did not appear to react significantly with carbohydrate moieties on trout IgM, as determined by the failure of proteolytic digests of trout IgM to inhibit in radioimmunoassay against intact trout IgM. Membrane immunoglobulin determinants were detectable on the membrane of both thymocytes and splenocytes with the lactoperoxidase-catalyzed iodination reaction. Radioimmunoassay of lysates of lymphocytes confirmed the association of immunoglobulin with trout thymocytes. By radioimmunoassay, lysates of thymic and splenic lymphocytes contained similar amounts of immunoglobulin, equivalent to approximately 8 X 10(4) molecules of IgM per cell (calculated as the monomer micro 2L2).