In this communication we demonstrate that B cells from CBA/N mice with x chromosome-linked immunodeficiency and B cells from newborn mice, that have very limited ability to present antigen to antigen-specific T cell lines, acquire this function following preincubation with IL-7 or IL-10. These interleukins do not affect the antigen presenting function of B cells from normal, adult CBA mice. Incubation of B cells from xid and newborn mice with IL-7 or IL-10 but not with IL-2 induces expression of Lyb-5 marker on these cells. The study shows that the property of IL-7 and IL-10 to induce antigen presenting cell (APC) activity in immature B cells is associated with appearance of Lyb-5 antigen on these cells.