Antigenic competition was studied on reciprocal combinations of non-cross-reacting erythrocyte (RBC) antigens, i.e., sheep erythrocytes (SRBC), horse erythrocytes (HRBC), and chicken erythrocytes (CRBC), as competing antigen or as test antigen. Unlike SRBC and HRBC, CRBC did not induce antigenic competition, and mice previously injected with CRBC showed normal direct and indirect hemolysin plaque-forming cell (PFC) responses to the other RBC subsequently injected. Formaldehyde-treated (F) SRBC that retained the capacity to prime mouse for secondary anti-SRBC antibody response but lost the capacity to elicit primary antibody response also could not induce antigenic competition. CRBC or F-SRBC, i.e., non-competing antigen, included with SRBC interfered with the competition-inducing effect of the latter RBC. When mice that had been injected with SRBC together with either one of the non-competing antigens were subsequently immunized with HRBC their anti-HRBC PFC responses were significantly restored, whereas mice pre-injected with SRBC alone showed markedly suppressed PFC responses to HRBC. Although the non-competing antigen was found not to affect both the direct and indirect PFC responses to the simultaneously injected SRBC, the response of the immune system to the non-competing antigen should alter the concurrent response to SRBC, and probably should prevent the generation of the suppressor thymus-derived (T) lymphocytes, allowing the generation of the helper T lymphocytes for IgM and IgG anti-SRBC antibody formation.