Helper T cells activate a set of lymphokine genes upon recognition of antigens presented in the context of the major histocompatibility complex on antigen presenting cells (Arai et al., 1986; Miyajima et al., 1988). Activation of T cells proceeds in two distinct stages. The flrst step is triggered by binding of an antigen to the T cell antigen receptor/CD3 complex that leads to the activation of protein kinase C (PKC) and an increase in intracellular Ca2+. This step, which is substituted by phorbol ester and calcium ionophore (Weiss et al., 1984), possibly proceeds through GTP binding protein...