Tyrosine kinase inhibitors, such as imatinib, have dramatically improved the outcomes for patients with selected cancers. For imatinib, western blotting of phospho-CrkL was an insensitive, indirect, and descriptive method to determine drug efficacy. Greater use of targeted therapies should involve more quantitative evaluation of the target's dose-inhibition. The Src/Abl kinase inhibitor dasatinib has recently been approved for use in Ph+ leukemias after failure with imatinib. Src family kinases (SFK) also play a critical role in nonhematologic cancers. We have developed a flow cytometric assay to measure SFK autophosphorylation levels in blood mononuclear cells and observed a direct correlation between its inhibition and patient dosage. This method provides a sensitive, quick, and quantitative tool to assess drug efficacy.