STIMULATION of T lymphocytes through their antigen receptor (T-cell receptor; TCR) results in the activation of a tyrosine kinase^1,2 and the generation of phosphatidyl inositol (PtdIns)-derived second messengers^3–5. Several reports have indicated that CD45, a haematopoetic cell-specific surface glycoprotein with tyrosine phosphatase activity in its cytoplasmic domain^6–10, is important in lymphocyte activation^11–14. To examine the possibility that CD45 might influence proximal signal transduction events through the TCR, we have isolated a variant of the human T-cell leukaemic line, HPB-ALL, which fails to express this phosphatase. Unlike cells expressing CD45, stimulation of the TCR in the CD45-negative cell does not result in PtdIns-derived second messengers. Reconstitution of CD45 expression restored early signalling events through the TCR. To localize the site of CD45 action, the human muscarinic type 1 receptor, which also activates the Ptdlns second messenger pathway^15,16, was transfected into the CD45-negative cell. Although stimulation of the TCR failed to generate Ptdlns-derived second messengers, there was normal activity of the PtdIns pathway when human muscarinic receptor type 1 was stimulated, despite the absence of CD45. These data indicate that CD45 influences a cellular component that is essential for effective coupling of the TCR to the PtdIns second messenger pathway.