SummaryA high‐titre IgE‐antibody response to Schistosoma japonicum antigen (Sj) is obtained if helper T cells are appropriately induced by the infection. The antibody responses were found to be regulated by a gene(s) linked to the major histocompatibility complex. Strains of H‐2aand H‐2khaplotypes were high responders, whereas H‐2d, H‐2band H‐2sstrains were not. Results obtained by using intra‐H‐2 recombinant strains suggested that the immune‐response (Ir) gene was mapped in the I‐E subregion, although a possibility of the presence of complementary genes was not excluded. Interestingly, in low responders such as BALB/c mice, Sj‐specific carrier effect could be demonstrated when mice were infected with S. japonicum before challenge immunization with dinitrophenylated Sj (DNP‐Sj). Moreover, BALB/cas well as C3H/He was a high responder to Sj in proliferation of peritoneal exudate T‐lymphocyte enriched cells in vitro, whereas B10.S was not. Based on these observations, possible roles of Ir genes in immune re