p56lck is a lymphoid cell-specific member of the src family of cytoplasmic tyrosine kinases. In helper and cytotoxic T cells it is physically associated with the CD4 and CD8 surface antigens and appears to play a role in signal transduction during T-cell activation. p56lck contains both an SH3 and an SH2 Src homology domain. Such domains have been suggested to play a role in the regulation of the activity or function of both receptor and non-receptor tyrosine protein kinases. Deletion of either or both domains in p56lck was found here to activate the protein and to lead to increased phosphorylation of the autophosphorylation site, Tyr-394, in vivo. These findings are consistent with the hypothesis that these domains participate in repression of the kinase activity of p56lck. None of the deleted forms was capable of transformation of fibroblasts. Deletion of the SH3 domain of a constitutively activated form of p56lck, p56lckF505, did not diminish the transforming activity of this protein. This suggests that this domain is dispensable for the transformation of fibroblasts by p56lck. In contrast, deletion of the SH2 domain abolished the transforming potential of activated p56lckF505. However, interpretation of this effect is made somewhat difficult because the mutation also lowered the steady-state abundance of the protein.