Antibody formation by transferred hyperimmune spleen cells has been studied with bovine serum albumin as antigen and inbred mice as donors and recipients. It has been shown that isologous adult and neonatal hosts support antibody formed in neonatal hosts was less than that in adult recipients. Antibody was not formed in homologous adult or newborn recipients and this is most likely because of rejection of the transferred cells prior to their formation of antibody. The host vs. graft reaction could be suppressed by prior x-irradiation of recipients with consequent antibody formation by transferred homologous cells. Further, if isologous recipients were irradiated prior to cell transfer, the antibody response was found to be greater than that in nonirradiated isologous hosts. Transferred cells appeared to survive in the isologous recipients for a prolonged period of time with continuing formation of antibody.