Evidence is presented that mice with X-linked immunodeficiency (xid) express strong Mlsa,d determinants, a putative marker of the mature subset of B cells. Although young (3-5 wk) (CBA/N X DBA/2)F1 male (xid+) mice stimulated only very weak mixed lymphocyte reactions (MLR) to Mlsa,d determinants, older mice (greater than 7 wk) regularly elicited conspicuous responses, despite being totally unresponsive to TNP-Ficoll. Expression of Mlsa,d determinants by xid+ mice was also detected by the procedure of negative selection in vivo. Thus BALB/c T cells were totally depleted of Mlsa,d reactivity after blood to lymph recirculation through 10-wk old irradiated xid+ (CBA/N X DBA/2)1 male mice. Significantly, a marked (90%) reduction in the anti-Mlsa,d response also occurred with T cell filtration through 3-wk xid+ mice, i.e., mice that elicit only minimal primary MLR; filtration through 3-wk xid- normal female mice led to near-complete (99%) negative selection. Collectively these data indicate either, (a) that xid+ mice contain appreciable numbers of cells with at least some of the properties of mature B cells, or (b) that the expression of Mlsa,d determinants is not restricted to mature B cells. In either case, B cells from xid mice cannot be viewed as a simple model for immature normal B cells.