The glycolytic enzyme enolase (2-phosph-D-glycerate hydrolyase) exists as several dimeric isoenzymes (aa, ab, ay and yy) composed of three distinct subunits, a, b, and y. Three isoenzymes are found in human brain: aa, ay and yy. The ay and yy-enolase isoenzymes are also known as neuron-specific enolase (NSE) as these isoenzymes initially were detected in neurons and neuronendocrine cells. The NSE levels are low in health and benign subjects. Elevated levels are commonly found in patients with malignant tumors with neuronendocrine differentiation, especially small cell lung cancer and neuroblastoma. Lung cancer is one of the most spread cancer forms with incidences about 50~100 per 100,000 population. Approximately 20% of the lung cancer is small cell lung cancer. Patients with small cell lung cancer show various proportions of ay and yy isoenzyme. The determination of NSE should detect and y isoforms with the same sensitivity (1). The antibodies for this particular assay are specific f