The frequency of B cells specific for phosphorylcholine (PC) in spleens of nonimmune conventionally-reared and germfree adult Balb/c mice was determined by an in vitro splenic focus technique. Analysis of spleen cells from 11 conventionally reared donors revealed an average of 19.1 PC-specific clonal precursor cells/10⁶ donor B cells when calculated utilizing a cloning efficiency previously determined for the in vitro splenic focus technique of 4%. This number is one-tenth the frequency determined for 2,4-dinitrophenyl- and 2,4,6-trinitrophenyl-specific B cells; however, both idiotypic and isoelectric analysis reveal that the majority of B cells specific for PC yield monoclonal antibodies which share the TEPC 15 idiotype and contain a homogeneous IgM peak with a pK of 4.60. Thus this single clone appears on the average to be represented by more than 10³ B cells in conventionally reared nonimmune adults. A similar frequency for precursor cells of this clonotype was found in an analysis of seven germfree adult mice so that the high frequency of B cells of this clonotype appears to be independent of previous antigenic stimulation. Among the 11 conventionally reared donors analyzed, frequencies varied from 1.6 to 52/10⁶ B cells—an extreme degree of variation when compared to the almost invariant frequencies observed for 2,4-dinitrophenyl, 2,4,6-trinitrophenyl, and fluoroscein. This variation is interpreted as a reflection of the life cycle of a single clone with a possible role for antigen implied by smaller variations observed in germfree mice.