T- and B-cell participation in the immune response induced by immune ribonucleic acid (iRNA) preparations against T-dependent antigens was studied using athymic nude, neonatally thymectomized (NT) and cyclophosphamide-treated (CY) mice. The iRNA(T + B) preparations were made from the spleen of BALB/c mice immunized with these antigens. Injection of the iRNA into nude or NT mice caused an increase in the number of specific rosette-forming cells (RFC) and of memory cells capable of responding to secondary stimulus with a small dose of the corresponding antigen. Injection with T-dependent antigens or with iRNA(T + B) did not cause any immune response in CY mice, suggesting depletion of the B-cell function. The iRNA(T) and iRNA(B) were prepared, respectively, from the thymuses of BALB/c mice and from the spleens of nude mice which had been immunized with T-dependent antigens. Injection of nude mice with both iRNA(T) and iRNA(B) caused an increase in the number of specific RFC and the secondary antibody formation response after boosting with a small dose of the corresponding antigen. Injection of iRNA(T) preparation into nude mice could induce the anamnestic response after boosting. However, neither of the iRNA(T) or iRNA(B) preparation could induce in nude mice the proliferation of the number of specific RFC. These results indicate the presence of at least two kinds of iRNA preparations against T-dependent antigens and that the cooperation of iRNA(T) and iRNA(B) was required for the induction of immune response against T-dependent antigens.