In the present investigation the differences in the capacity of thymocytes (Th) and thymus-related peripheral blood lymphocytes (TBL) of the guinea-pig to form spontaneous rosettes with rabbit erythrocytes (RRBC) were further evaluated. Cytochalasin B (CB) inhibited TBL rosette formation but was largely ineffective with respect to Th. This suggests that membrane modulation and microfilament function are requirements for the formation of TBL but not of Th rosettes. A certain critical RRBC receptor density on the lymphocyte may be necessary for a stable rosette. Our results suggest that different RRBC receptor densities on Th and TBL account for the differences in rosette forming capacity of these two cell types. In native Th, the receptor density may be high enough to warrant stable rosette formation. In TBL, however, active membrane movement and perhaps clustering of receptors may be necessary to achieve the local critical receptor density as prerequisite of a stable rosette.