Himalayan rabbits of a closed colony were immunized with injections of heat-killed Salmonella typhi and antiidiotypic antibodies against the induced anti- S. typhi antibodies were produced in rabbits of the same colony as well as in random-bred rabbits. Rabbits of the closed colony showed no proliferative response in a mixed lymphocyte culture of peripheral blood. Antiidiotypic sera from Himalayan rabbits recognized the idiotype in the corresponding immunizing sera alone, while one of sera from random-bred rabbits showed a cross-reaction with 8 out of 10 anti-S. typhi sera of Himalayan rabbits but not with any of 10 anti-S. typhi sera of unrelated random-bred rabbits. The cross-reactivity of the antiserum remained intact after absorption with unrelated immune precipitates. With this antiidiotypic serum the immunizing serum formed a bimodial arc at beta-gamma mobility and all other anti-S. typhi sera containing the cross-reactive idiotype a single arc at beta mobility. Solubilized immune precipitates of cross-reactive idiotype-anti-idiotype reaction could bind radiolabelled lipopolysaccharide (LPS) from S. typhi, but this anti-LPS activity was not revealed in an isoelectric focusing analysis. Gel analysis showed that the cross-reactive idiotype was located mainly in the macroglobulin fractions. The idiotype in the serum of the immunizing rabbits diminished and then was undetectable following the 2nd and the 3rd immunizations. When 3H-TdR uptake was examined in a mixed cell culture of peripheral blood from immunizing and antiidiotypic rabbits, there was a fluctuation in the proliferative response with two peaks occurring at a 4-week interval. An analysis of such a proliferative response was carried out by separating leukocytes and plasma from blood of the immunizing and the antiidiotypic rabbits. Mixed culture of cells alone did not produce a proliferative response, while culture of cells from the immunizing rabbit together with antiidiotypic plasma resulted in a potent reaction, irrespective of the presence of plasma from the immunizing rabbit. Presence of cells from the antiidiotypic rabbit in the culture inhibited this proliferative response. A fluctuation in the proliferative response to antiidiotypic serum was also observed with peripheral blood leukocytes (PBL) from rabbits producing the cross-reactive idiotype, while the antiidiotypic serum did not stimulate cells from rabbits which had been similarly immunized with S. typhi but did not produce the idiotype. PBL from the immunizing rabbit where the idiotype production ceased following the tertiary immunization were found to suppress definitely the proliferative response induced by the cross-reactive idiotype-anti-idiotype reaction. The suppressive activity was lost in PBL from the same rabbit after a cortisone treatment and the following antigenic stimulation of the animal led to reappearance in the serum of the idiotype. These results support the immune regulatory model which involves idiotype-anti-idiotype interactions.