While Ig+ B cells appear to be the principal cell type expressing immunogenic minor lymphocyte stimulatory (Mls) determinants, both T cells and B cells are capable of mediating deletion of developing Mls-reactive thymocytes. In addition, levels of mouse mammary tumor proviral transcripts are increased after B or T cell stimulation, and expression of functional Mls determinants is augmented by activation of B cells. These findings suggest Mls determinants are present on B and T lymphocytes, and that activation of B and T cells augments Mls expression. In the present study, we wished to determine whether B and T cells were required for expression of Mls determinants by examining mice with severe combined immunodeficiency (SCID) containing no detectable Ig+ B cells or TCR+ T cells, as well as animals that expressed the X-linked immunodeficiency (xid) defect and lacked a subset of mature B cells. We found Mlsa-reactive V beta 6hi T cells were deleted from thymi of male (CBA/NxAKR/J)F1 xid mice, and that spleen cells from these animals stimulated anti-Mlsa mixed lymphocyte responses by unprimed B10.BR spleen T cells. In addition, Mlsc-reactive V beta 3hi AKR/J thymocytes and spleen T cells were deleted in AKR/J----SCID bone marrow chimeras, and spleen cells from SCID mice stimulated proliferation by an Mlsc-specific T cell clone. These results demonstrate that both xid mice and SCID animals express Mls determinants that mediate deletion of developing, Mls-responsive thymocytes and stimulate proliferation of mature, Mls-reactive T cells. Hence, mature B cells and T cells are not essential for Mls expression.