Heterologous erythrocytes have long been in general use as multideterminant antigens. The development of the hemolytic plaque assay has allowed the enumeration of individual cells producing antibody directed against an erythrocyte determinant (1). In the response of mouse spleen cells, it has been shown in vitro (2) and in vivo (3) that a portion of the plaque-forming cells (PFC) which arise to the sheep erythrocyte (SRBC) antigen also yields plaques or “cross-reacts” with the erythrocytes of the closely related species, goat (GRBC), but not with those of the phylogenetically more distant species, burro (BRBC). It is assumed that the extent of cross-reactivity of these PFC reflects to some degree the extent of cross-reactivity of the receptors on their bone marrow-derived precursors (B-cells). Immunization of mice with heterologous erythrocytes also leads to a rapid increase in the spleen in the number of thymus-derived lymphocytes (T-cells) with receptors specific for erythrocyte determinants (4, 5).