Utilization of molecular imaging of apoptosis for clinical monitoring of tumor response to anti-cancer treatments in vivo is highly desirable. To address this need, we now present ML-9 (butyl-2-methyl-malonic acid; MW = 173), a rationally designed small-molecule detector of apoptosis, based on a novel alkyl-malonate motif. In proof-of-concept studies, induction of apoptosis in tumor cells by various triggers both in vitro and in vivo was associated with marked uptake of (3)H-ML-9 administered in vivo, in correlation with the apoptotic hallmarks of DNA fragmentation, caspase-3 activation and membrane phospholipid scrambling, and with correlative tumor regression. ML-9 uptake following chemotherapy was tumor-specific, with rapid clearance of the tracer from the blood and other non-target organs. Excess of non-labeled "cold" compound competitively blocked ML-9 tumor uptake, thus demonstrating the specificity of ML-9 binding. ML-9 may therefore serve as a platform for a novel class of small-molecule imaging agents for apoptosis, useful for assessment of tumor responsiveness to treatment.