We wished to determine whether tolerized cells are able to process and present antigen based on our hypothesis that tolerized B cells be unable to function in the normal capacity as antigen-presenting cells if they are to remain the tolerant state. Results in this study show that the ability of the murine lymphoma model for immature B cells, CH31, to process pigeon cytochrome c was greatly down-regulated when cultured in the presence of rabbit anti-mouse IgM. In contrast, the same anti-IgM treatment had no significant effect on the antigen-presenting cell function of the lymphoma model for mature B cells, CH112. Presentation of CNBr-cleaved fragments of pigeon cytochrome c by either CH31 or CH12 cells was not affected by the antibody treatment. Furthermore, CH31 cells pre-incubated with pigeon cytochrome c were not subject to the anti-IgM inhibition of the antigen presentation. These observations suggest that pertubation of surface immunoglobulin molecules on CH31 immature B cells causes down-regulation of their antigen-processing machinery.