The majority of thymocytes in suspension (72.1%) formed nonimmune rosettes with guineapig erythrocytes but not with erythrocytes from sheep, cats, dogs, pigs, or man. In contrast, only a minority of cells form lymph node (2.1%) or spleen (1.4%) rosetted with guineapig erythrocytes. Treatment of guineapig erythrocytes with neuraminidase of 2-S-aminoethyl-isothiouronium bromide did not enhance rosette formation. Adherence of the erythrocytes to tissue sections was achieved in the thymic cortex and T-cell-dependent areas of lymph node and spleen. Absorbed equine anti-rat antithymocyte serum bound 98% thymocytes and 70.0, 35.1 and 44.6% lymphocytes from peripheral blood, spleen and lymph nodes respectively. Surface immunoglobulin was demonstrated on 2.2% thymocytes, 35% peripheral-blood lymphocytes and 43 and 51% cells from lymph nodes and spleen respectively, Complement receptors were determined in suspension with erythrocyte-antibody-complement complexes. Rosetting was observed in 44% splenic, 29% lymph node and 3% thymocyte cellular suspension. The presence of Fc receptors for IgG was assessed by determining the pattern of binding of erythrocyte-antibody complexes to frozen tissue sections. For rats, antithymocyte serum is the method of choice for T cells, whereas SIg determination is the most reliable B-cell marker.