The CD4 protein is expressed on a subset of human T lymphocytes that recognize antigen in the context of major histocompatibility complex (MHC) class II molecules. Using Chinese hamster ovary (CHO) cells expressing human CD4, we have previously demonstrated that the CD4 protein can mediate cell adhesion by direct interaction with MHC class II molecules. In T lymphocytes, CD4 can also function as a signaling molecule, presumably through its intracellular association with p56lck, a member of the src family of protein tyrosine kinases. In the present report, we show that p56lck can affect cell adhesion mediated by CD4 and MHC class II molecules. The expression of wild-type p56lck in CHO-CD4 cells augments the binding of MHC class II+ B cells, whereas the expression of a mutant p56lck protein with elevated tyrosine kinase activity results in decreased binding of MHC class II+ B cells. Using site-specific mutants of p56lck, we demonstrate that the both the enzymatic activity of p56lck and its association with CD4 are required for this effect on CD4/MHC class II adhesion. Further, the binding of MHC class II+ B cells induces CD4 at the cell surface to become organized into structures resembling adhesions-type junctions. Both wild-type and mutant forms of p56lck influence CD4-mediated adhesion by regulating the formation of these structures. The wild-type lck protein enhances CD4/MHC class II adhesion by augmenting the formation of CD4-associated adherens junctions whereas the elevated tyrosine kinase activity of the mutant p56lck decreases CD4-mediated cell adhesion by preventing the formation of these structures.