Limiting numbers of spleen cells from mice primed with influenza virus were transferred into lethally irradiated syngeneic recipients. Upon antigenic stimulation of fragment cultures of the recipient spleens it was observed that a linear relationship existed between the number of transferred spleen cells and the number of responding fragments. The antibody product of individual fragments exhibited a highly restricted heterogeneity in isoelectric focusing as well as in its reactivity against various viral antigenic determinants. It was concluded that the limiting cell type in this adoptive transfer system corresponds to the virus‐primed B cell which, upon antigenic stimulation, gives rise to a clone of antibody‐producing cells. PR₈‐specific precursor B cells occurred at a frequency of at least 1/4000 splenic B cells in PR₈‐primed BALB/c mice. Approximately 25% of the stimulated cell clones produced sufficient quantities of antibody (200 ng) to render feasible an analysis of closely related viral antigens.