Altered expression of cell surface Ag is an early event accompanying Ag-, mitogen-, or phorbol ester-induced activation of mature T cells. In this report, phorbol ester-induced changes in the expression of several functionally significant cell surface molecules are explored on immature thymocytes and lymphoma cells presenting a cortical CD4+8+ double-positive (DP) phenotype. Both CD4 and CD8 expressions are down-modulated on DP cells incubated with PMA. Cell-surface expression of CD4 and CD8 remains depressed for 72 h in the presence of PMA, but is restored after removal of PMA from the culture medium. The PMA-mediated loss of Ag expression is associated with a rapid down-regulation of steady state CD4 and CD8 mRNA transcript levels in treated cells. The sustained loss of CD4 and CD8 surface expression on DP cells is a selective event because CD5 expression is enhanced, H-2 expression is unchanged, and CD3 expression is only transiently diminished by PMA stimulation. Other T cell-activating agents, including Con A, ionomycin, and anti-CD3 mAb, induce selected surface antigenic changes on DP cells, but do not mimic the pattern of altered Ag expression observed after PMA stimulation. These data demonstrate that, similar to mature subsets, T cells in the DP nonmature compartment undergo alterations in expression of functionally important cell-surface molecules in response to activating agents. Nevertheless, distinctions between mature and nonmature T cells regarding specific alterations in differentiation Ag phenotype suggest that the effect of PMA on expression of these molecules depends, in part, on the maturation stage of the target cell population.