Lewis strain rat lymphocytes were exposed in vitro to a variety of specific and nonspecific blastogenic stimuli. The surfaces of the transformed lymphocytes were examined by indirect immunofluorescence for the presence of T cell antigens and immunoglobulin molecules. More than 90% of lymphocytes that underwent blast transformation after exposure to foreign histocompatibility antigens (mixed lymphocyte reaction; in vitro allograft reaction), purified tuberculin, phytohemagglutinin (PHA), and concanavalin A (Con-A) had T cell antigenic markers on their surfaces. 70-92% of the antigen-stimulated blast cells also had readily detectable surface immunoglobulin molecules, whereas less than 3% of the PHA- and Con-A-activated cells were Ig(+). Pokeweed mitogen (PWM) appeared to activate both B and T cells, but the T cells did not have detectable surface immunoglobulin molecules. Nonactivated control cultures contained T(+)Ig(-) lymphocytes almost exclusively. The results suggest that thymus-dependent rat lymphocytes express increased amounts of detectable immunoglobulin on their surface in response to specific stimulation with antigen. It is postulated that the acquisition of immunological competence by activated T cells may be related to this expression of surface immunoglobulin.