Using the clonable pre-B cell assay, we have identified B cell progenitors that are not yet committed to the production of a particular H chain allele. These cells represent approximately 10-20% of clonable pre-B cells found in 15-d fetal liver. The clonable pre-B cell assay provides an environment adequate for the expansion and differentiation of these cells into mature, Ig-secreting, cells. Using the same methodology, we have also identified progenitors that are uncommitted to the production of a particular L chain isotype. Moreover, investigating the growth requirements for clonable pre-B cells has led to the discovery of selective growth conditions that distinguish cells before and after commitment to H chain allotype.