We examined the role of L3T4 and LFA-1 molecules in T-helper-cell activation, under conditions where the physical stability of T helper-accessory cell interactions was not an issue. T-helper hybridomas were activated by accessory cells coated either with concanavalin A (Con A) or with CD3 antibodies. Activation of the T helper cells was measured by microtubule-organizing center (MTOC) reorientation as an early activation event, and by interleukin-2 (IL-2) production as an indication of a fully matured response. Both parameters were strongly blocked by L3T4 and LFA-1 antibodies in the case of Con A activation. In the case of stimulation with accessory cell-bound CD3 antibody, activation was blocked by LFA-1 but not L3T4 antibody. These results support the notion that L3T4 and LFA-1 molecules play more than a simple adhesion role in T-cell activation. The differential effect of L3T4 antibody in the case of Con A activation vs CD3 activation is consistent with the possibility that L3T4 and the alpha/beta portions of the T-cell receptor must interact during antigen- and lectin-stimulated T-cell activation.