Tyrosine phosphorylation of several intracellular proteins is observed very early during T cell activation. p56lck, a non-receptor src-like protein tyrosine kinase (PTK) which is associated with the intracellular domains of CD4 and CD8 co-receptors, has been implicated in these early signal transduction events. Furthermore, recent experiments indicate that the receptor phosphotyrosine phosphatase, CD45, might be important in the regulation of p56lck PTK activity and that its expression is required for the generation of second messenger molecules following TCR triggering. Here, using co-capping experiments and double indirect immunofluorescence microscopy in functional human T lymphocytes, a specific co-distribution of a significant fraction of p56lck with CD45, but not with several other cell surface proteins, has been revealed. This is the first demonstration of a physical interaction between a receptor phosphotyrosine phosphatase and a PTK under physiologically relevant conditions. In addition, after antibody-induced capping of CD4, both a co-localization of p56lck and CD4, and concomitantly a significant increase in intracellular phosphotyrosine at the sites of CD4 caps were observed. In strong contrast to these results, co-clustering of CD4 with CD45 did not result in any detectable intracellular phosphotyrosine at the cap sites. These data indicate that CD45 can act on CD4-associated phosphoproteins in viable human T lymphocytes. Further, this provides evidence that p56lck PTK is a substrate of CD45 phosphotyrosine phosphatase in vivo and thereby supports the idea that CD45 is an early regulator of T cell activation involved in the modulation of the coupling of receptor-triggered events to intracellular signalling pathways.