Human helper T lymphocytes can be dissected into two functionally distinct subpopulations based on expression of the CD45RA (2H4) or CD45R0 (UCHL-1) surface antigens. While both subpopulations are able to induce equivalent levels of B cell activation and proliferation, only the CD4+CD45RA- subpopulation is capable of inducing B cell differentiation in pokeweed mitogen (PWM)-stimulated cultures. To define the mechanism responsible for the dichotomy between induction of proliferation and differentiation by the two CD4+ subpopulations, we examined the abilities of the purified T cell subpopulations to produce lymphokine mRNA following T cell activation. Northern analysis revealed that both subpopulations produced interleukin (IL) 2 and interferon (IFN)-gamma mRNA following PWM activation. The CD4+CD45RA- subpopulation, however, produced higher levels of IFN-gamma mRNA and the CD4+CD45RA+ cells produced higher levels of IL 2 mRNA. Neither subpopulation elaborated detectable mRNA for IL 4, IL 5 or IL 6. Of greatest significance was that the addition of recombinant or T cell-derived lymphokines could not compensate for the inability of the CD4+CD45RA+ subpopulation to induce B cell differentiation in PWM assays. Direct T-B cell contact was required for the optimal induction B cell differentiation in these assays, suggesting that CD4+CD45RA+ T cells were deficient in their ability to directly deliver the T cell-B cell signals required for B cell differentiation. These results suggest that the differential ability of the two subpopulations of CD4+ T cells to induce B cell differentiation does not result from differences in lymphokines elaborated, but may result from differences in their abilities to interact directly with B cells to initiate differentiation.