The relationship between precursors for B1 and B2 cells remains controversial. On the basis of recent experimental results, it is highly probable that B1 cells can be generated from the same sources as B2 cells. However, the precursor cell types involved, the microenvironment and the age of donor and recipient may all determine whether B1 cells are generated under the variety of experimental conditions that have been described in the literature. Definition of these influences will be important in understanding the role of B1 cells in autoimmune disease. Further differences in antigen responsiveness, localization and signaling between B1 and B2 cells will also be informative with respect to their roles in antibody production. © 1993.