The role of tyrosine phosphorylation in cell transformation has been well established. It has been proposed that protein tyrosine phosphatases (PTPases) may be capable of dephosphorylating critical substrates involved in the transformation process, suggesting that they represent a tumor suppressor family of enzymes. Indeed, recent work showed that overexpression of some PTPases in malignant cells counteracted the action of oncogenic tyrosine kinases although overexpression of other forms of these enzymes increased tumorigenicity. The work described herein has provided some insight into the action, both antagonistic and synergistic, of the kinases and phosphatases on cell growth and transformation.