Signal transduction through both cytokine and lymphocyte antigen receptors shares some common pathways by which they initiate cellular responses, such as activation of mitogen-activated protein kinase(s)(1,2). However, other signalling components appear to be uniquely coupled to each receptor. For example, the interferon receptors transduce regulatory signals through the JAK/ STAT pathway, resulting in an inhibition of growth and of antiviral effects, whereas this pathway apparently plays no role in T-cell-receptor (TCR)-dependent gene expression(3,4). Conversely, signal transduction through the TCR requires the tyrosine kinases Lck and ZAP-70 and the tyrosine phosphatase CD45 (ref. 5). Here we show that, unexpectedly, transmission of growth-inhibitory signals by interferon-alpha (IFN-alpha) in T cells requires the expression and association of CD45, Lck and ZAP-70 with the IFN-alpha-receptor signalling complex.