Almost all B cells in autoimmune mice with the viable motheaten (me^v) mutation express the Ly-1 cell surface antigen, which marks a minor population of B cells constituting a separate lineage in normal mice. Immunoglobulins primarily of the M and G3 classes, which in both normal and me^v mice contain high levels of lambda light chain, are produced in excess in me^v mice. These and other observations suggest that the development of B cells that express Ly-1 is regulated independently from the development of B cells that do not express Ly-1. B cells bearing the Ly-1 surface antigen may play specialized roles in the normal immune system and in autoimmunity by regulating other B cells via lymphokines, by producing antibodies to self and certain foreign antigens, and by preferentially secreting immunoglobulin M and immunoglobulin G3.