It has previously been reported (Hiinig, T. et al., Eur. J. Zmmunol. 1983. 13: 1) that highly purified peripheral T cells do not respond to concanavalin A (Con A) even in the presence of Con A-induced spleen cell supernatant as a source of interleukin 2 (IL 2). In the present report, the hypothesis was tested whether this unresponsiveness correlates with the observed inability of Con A to mediate cell-cell contact between highly purified T cells. It was found that T cells, pretreated with neuraminidase to reduce their net negative charge, were both aggregated and rendered IL 2-reactive by Con A. In addition, leukoagglutinin (LA) was found to be able to both agglutinate untreated T cells and to make them IL 2-reactive. Neuraminidase treatment reduced the concentration of LA required to mediate both effects. Neuraminidase treatment did not alter the overall Con A-binding capacity of T cells, nor did it induce reactivity to IL 2 in the absence of lectin. Furthermore, irradiated, neuraminidase-treated T cells served as accessory cells (AC) in the induction of responsiveness to IL2, but not for production of IL2, which depends on Ia' AC. Finally, Lyt-2-neuraminidasetreated peripheral T cells responded in the same fashion as whole T cell populations, indicating that at least some T cells of the helper phenotype need not interact with Ia'AC for the induction of IL 2 responsiveness.