A subpopulation of lymphocytes is defined, which—under conditions used for Isolating mono-nuclear cells from peripheral blood by the Isopaque-Ficoll gradient—is lost in the erythrocyte pellet because of rosette formation with the autologous erythrocytes in the gradient. Reapplication of the resuspended erythrocyte pellet on a new gradient with higher density could, however, recover the subpopulation. Lymphocyte surface marker analysis on such cell showed that they were enriched in T cell (judged by rosette formation with sheep erythrocytes) and in auto-rosette-forming cells. The importance of these findings is discussed in relation to the wide use of the Isopaque-Ficoll method.