In T lymphocytes, CD2 forms part of a loosely associated membrane complex which includes the T cell receptor (TcR) for antigen, the CD3 subunits, CD4 or CD8, CD5 and the protein tyrosine kinases p56lck and p59fyn. The interaction of CD2 with tyrosine kinases in this complex provides a possible mechanism for transmembrane signal transduction by CD2. We have investigated whether the interaction of CD2 with the kinases is dependent on other known members of the complex, or whether an independent association can be observed. Using in vitro kinase assays with immune complexes precipitated from cell lysates, we demonstrate that CD2 can associate with p56lck and p59fyn in a rat thymoma line that does not express CD4 or CD8, and in a TcR-negative Jurkat cell line. In TcR-positive Jurkat cells that express rat CD2, interaction of CD2 with p56lck and p59fynWas clearly seen, but it was absent in cells where the cytoplasmic tail of CD2 is truncated, indicating that the interactions are mediated by the cytoplasmic region of CD2. Furthermore, using cells expressing CD2 molecules with partial truncations in the cytoplasmic domain, we show that the association of CD2 with p56lck: is progressively lost as the cytoplasmic domain is shortened, and that the capacity of the mutants to associate with p56lck correlates with their capacity to transduce transmembrane signals.