Antigenic encounter by T cells induces immunological synapse formation and T-cell activation. Using different concentrations of toxic shock syndrome toxin-1 (TSST-1) as stimulus, we examined the capacities of dendritic cells (DC) and macrophages (Mphi) to prime syngeneic naive T cells. DCs were, under all experimental settings, more efficient than Mphi at clustering T cells. Translocation of the T-cell receptor (TCR) to the contact area was found to be induced by DCs, as well as by Mphi, in an antigen-dependent manner, although Mphi were less efficient at inducing TCR translocation. Capping of protein kinase C theta (PKCtheta) was also antigen dependent but induced exclusively by DCs. Likewise, DCs were found to be more potent inducers of interleukin-2 (IL-2) production and proliferation of naive T cells than Mphi. After 3 days of culture, DCs presenting 100 ng/ml TSST-1 induced interferon-gamma (IFN-gamma)-secreting cells, whereas Mphi did not. After 7 days of culture, DCs presenting 0.1 ng/ml TSST-1, and Mphi presenting high (as well as low) doses of TSST-1, induced IL-4-producing cells. We therefore provide evidence to show that antigen dose, type of antigen-presenting cell and time of differentiation can contribute to T-cell differentiation.