In this paper, data are presented which indicate that receptors of human peripheral blood lymphocytes (PBL) for pig erythrocytes (PRBC) and sheep erythrocytes (SRBC) are identical or at least very similar to each other with a higher avidity of PBL for SRBC than for PRBC. As an average, 50 +/- 9% of human PBL formed PRBC rosettes (Ep), compared to 70 +/- 7% of SRBC rosettes (Es). Lymphocytes incubated with SRBC plus PRBC ('co-rosetting') formed 10 +/- 5% of mixed rosettes (Em) but no pure Ep. Resuspension of rosetted lymphocytes followed by co-rosetting with the other erythrocyte species revealed that the majority of Ep became Em, while the majority of Es remained pure Es even after co-rosetting with PRCB; no pure Ep was newly formed. Incubation at 37 degrees C abolished almost all Ep as compared to 9% Es. Anti-thymocyte serum inhibited both types of rosette formation. PBL from patients with chronic lymphatic leukemia formed only 2.5 +/- 2.5% Ep. 1.7 +/- 1.1% of PBL expressed both membranes immunoglobulins and receptors for PRBC. Trypsin treatment abolished both kinds of rosette formation almost completely, while neuraminidase treatment of erythrocytes led to a raise in Ep formation from a mean of 53% to a mean percentage of 68%. Electron microscopic investigations did not reveal differences between binding sites of lymphocytes for PRCB and those for SRBC; however, with a technique different from the conventional one, intimate contact areas were observed between lymphocytes and both kinds of red blood cells.