Pp56lck is a member of the src family of tyrosine kinases mainly expressed in T lymphocytes. Src tyrosine kinases havebeen Implicated in the control of cell growth and differentiation in different cell types, but the mechanism of regulation of these enzymes is poorly understood. In order to characterize the distinct species of pp56lck, we have produced highyields of enzymatically active wild type pp56lck using the eukaryotic baculovirus expression system in Spodopterafrugiperda insect cells (Sf9). We find that the various species of baculoviral pp56lck are not only differentially phosphorylated (on serine and tyrosine residues) but also heterogeneously myristylated. Surprisingly a non-myristylated, very active form of bv-pp56lck is found in the cytoplasm of Sf9 cells. Fractlonation of T cells reveals that cytoplasmic pp56lck exists in T lymphocytes as well.