The enzymatic activity of the lymphocyte-specific tyrosine protein kinase p56lck appears to be tightly regulated by phosphorylation of the conserved carboxy-terminal tyrosine residue 505. Indeed, substitution of this tyrosine residue by a non-phosphorylatable phenylalanine results in a constitutively activated version of p56lck that can transform rodent fibroblasts. In this report, we evaluate the functions of the conserved non-catalytic Src homology (SH) domains 2 and 3 of p56lck in the regulation of its enzymatic activity in NIH3T3 fibroblasts. We found that deletion of the SH2 or, to a lesser extent, the SH3 domain of p56lck resulted in an increase in the tyrosine protein kinase activity of wild-type Lck polypeptides. The SH2 domain (but not the SH3 domain) was also required for full oncogenic transformation by Lck molecules activated through removal of tyrosine 505. This effect did not appear to be the result of a diminution of the enhanced catalytic activity of F505 Lck polypeptides. However, it may relate to the findings that the SH2 domain can bind and possibly enhance phosphorylation of specific phosphotyrosine-containing proteins. Taken together, these observations imply roles for the non-catalytic SH2 and SH3 domains in the regulation of the catalytic activity of p56lck. They suggest that the enzymatic function of this Src-related polypeptide is physiologically repressed by processes dependent on the presence of the SH2 and SH3 sequences. Moreover, they indicate that the SH2 domain also plays a positive role in the function of activated p56lck molecules in NIH3T3 cells.