Finding noninvasive methods to discern which patients' tumors bear a specific target molecule, and are presumably more likely to respond, remains a critical challenge. An anti-phospho-tyrosine antibody was labeled with indium ((111)In) using ethylenedicysteine (EC) as a chelator ((111)In-EC-P-Tyr). We hypothesized that tumor phosphokinase activity would be discernible by imaging with (111)In-EC-P-Tyr. A xenograft of A431 cells, a human epithelial carcinoma cell line overexpressing epidermal growth factor receptor (EGFR), was employed. Biodistribution studies confirmed increased tumor/muscle ratios of (111)In-EC-P-Tyr in the A431 model. Imaging demonstrated that a marked decrease in tumor uptake of (111)In-EC-P-Tyr occurred after 3 d of gefitinib therapy in A431 cells (gefitinib-sensitive), but not in H441 cells (gefitinib-resistant). Our results indicate that (111)In-EC-P-Tyr can detect tumor phospho-tyrosine kinase activity in animal models. This type of agent merits investigation in the clinic to determine if it can predict patient responses to kinase inhibitors based on phosphokinase imaging.