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Tyrosine protein kinases, in addition to their roles as viral transforming proteins and growth factor receptors, have been suggested to have specialized functions in tissue specific processes and in differentiation. High levels of soluble tyrosine kinases have been found in human serum and plasma. To determine if the level of tyrosine kinase activity is developmen- tally expressed in human serum, we assayed sera from 214 individuals of different ages from newborns to 90 years. We found that serum tyrosine kinase levels are high in newborns and the levels closely parallel skeletal growth until late adolescence. The serum tyrosine kinase levels increase again corresponding to the second and third decades and decline by the fourth decade of life. These studies show that tyrosine kinase levels are developmentally expressed in human serum and delineate the stages in post­natal development when changes in expression occur.