Transgenic mice carrying the proto-oncogene c-fos under the control of H-2Kb promoter (c-fos mice) were generated from (C57BL/6 x SJL)F2 mice. One line was backcrossed with C57BL/6 mice for 10 generations. These semi-congenic c-fos mice express exogenous c-fos RNA in their hematopoietic tissues. The following immunological states are apparent. (i) Titers of serum IgM, IgG, and IgA of naive c-fos were within the control range. (ii) These mice could not produce primary IgG antibody specific for immunized antigen. (iii) Production of primary IgM and IgA antibody to the antigen was within the control range. (iv) There were no IgG memory B cells generated in the spleens of c-fos mice. (v) Activities of antigen-presenting cells and carrier-specific helper T cells from c-fos mice were normal. These findings strongly suggest that the immune abnormality of c-fos mice is in limiting B cell function to the production of IgG to immunized antigens.