Phosphotyrosine cannot be detected on normal human ABL protein-tyrosine kinases, but activated oncogenic forms of the human ABL protein are phosphorylated on tyrosine in vivo. Activation of ABL can occur by substitution of the ABL first exon with breakpoint cluster region (BCR) sequences or by deletion of the noncatalytic SH3 (src homology region 3) domain. An alternative mode for the activation of the ABL kinases is hyperexpression at greater than 500-fold over endogenous levels. This is not a consequence of transphosphorylation of the hyperexpressed ABL molecules. ABL proteins translated in vitro lack phosphotyrosine, but tyrosine kinase activity is uncovered after immunoprecipitation and removal of lysate components. The rates of dephosphorylation of ABL and BCR-ABL fusion protein by phosphotyrosine-specific phosphatases are approximately the same. These combined results indicate that inhibition of ABL activity is reversible and suggest that a cellular component interacts noncovalently with ABL to inhibit its autophosphorylation.