The Ly-5 system is a composite family of molecules with different molecular weight. Each molecular form is characteristic of a different lymphoid population, in particular a 175,000- to 185,000-dalton doublet is peculiar of thymocytes and peripheral T cells. Athymic nude mice, however, lack the 185,000-dalton form on their lymphocytes. To investigate whether the expression of the 185,000 daltons requires the presence of thymus, we induced nude mice lymphocytes to differentiate into mature T cells in vivo by thymus grafting and, in vitro, into T cytotoxic cells (CTL) by cocultivation with alloantigen plus recombinant interleukin 2 (IL-2). The appearance of the 185,000-dalton form on lymphocytes of thymus-grafted nude mice but not on in vitro activated cytotoxic cells indicates an active role of thymus in the induction of this molecule and suggests that 185,000-dalton form could be a marker of intrathymic differentiation but not of the cytotoxic activity of T cells.