BACKGROUND: Acoustic sensors that exploit resonating quartz crystals directly detect the binding of an analyte to a receptor. Applications include detection of bacteria, viruses, and oligonucleotides and measurement of myoglobin, interleukin 1beta (IL-1beta), and enzyme cofactors. METHODS: Resonant Acoustic Profiling was combined with a microfluidic lateral flow device incorporating an internal reference control, stable linker chemistry, and immobilized receptors on a disposable sensor "chip". Analyte concentrations were determined by analyzing the rate of binding of the analyte to an appropriate receptor. RESULTS: The specificity and affinity of antibody-antigen and enzyme-cofactor interactions were determined without labeling of the receptor or the analyte. We measured protein concentrations (recombinant human IL-1beta and recombinant human myoglobin) and quantified binding of cofactors (NADP+ and NAD+) to the enzyme glucose dehydrogenase. Lower limits of detection were approximately 1 nmol/L (17 ng/mL) for both IL-1beta and human myoglobin. The equilibrium binding constant for NADP+ binding to glucose dehydrogenase was 2.8 mmol/L. CONCLUSIONS: Resonant Acoustic Profiling detects analytes in a relatively simple receptor-binding assay in <10 min. Potential applications include real-time immunoassays and biomarker detection. Combination of this technology platform with existing technologies for concentration and presentation of analytes may lead to simple, label-free, high-sensitivity methodologies for reagent and assay validation in clinical chemistry and, ultimately, for real-time in vitro diagnostics.