The activation of resting T cells to interleukin 2 (IL-2) production and DNA synthesis via Ti-CD3 is dependent on accessory cells (AC). Using positively selected, resting T cells activated with particle-bound anti-CD3, we investigated the ability of various cell lines to function as AC. We found that cell lines able to act as AC all expressed LFA-3, while cell lines not expressing LFA-3 were unable to provide AC signals. This applied to CD3+, CD4+, and CD8+ T cells. Sheep red blood cells (SRBC), which express LFA-3-like molecules, also had a weak, but significant AC function in this test system. Both CD4+ and CD8+ T cells activated with particle-bound anti-CD3 could be induced to enter DNA synthesis in the absence of AC when monoclonal antibodies reacting with CD2 were present instead of AC. IL-2 production could be detected in the latter cultures but not when positively selected CD3+ or CD2+ T cells were cultured alone. Our data suggest that activation of resting T cells via CD3 will lead to IL-2 receptor expression, while the interactions between LFA-3 and its ligand CD2 provide the necessary secondary signals for IL-2 production and induction of DNA synthesis.