B cells from normal DBA/2 and autoimmune NZB mice were transferred into H-2-compatible xid recipients where they engrafted without irradiation or other manipulation of the host. The properties of these cells and their interaction with the host environment were analyzed at the single cell level with a splenic focus assay. When similar numbers of NZB and DBA/2 anti-DNA-producing B cell precursors were transferred, they expanded at similar rates in xid recipients. The rate of expansion varied with the strain of the recipient: it was fastest in autoimmune-prone NZB . xid and slowest in DBA/2 . xid hosts. Cells producing antibodies reactive with the autoantigen DNA proliferated substantially faster than those reactive with the non-autoantigen trinitrophenylated keyhole limpet hemocyanin. These results suggest that 1) B cells from NZB mice do not behave differently from DBA/2 B cells, 2) the internal milieu of the recipient into which the cells are transferred has an important effect on B cell proliferation, and 3) B cells capable of autoantibody production may have a selective growth and/or differentiation advantage relative to other B cells.