The glycolytic enzyme enolase (2-phospho-D-glycerate hydrolase, EC 4. exists as several dimeric isoenzymes (aa, aß, a?, ßß and ??) composed of three distinct subunits a, ß and ?. The ? unit is found either in a homologous ??- or in a heterologous a?-isoenzyme and is known as neuron-specific enolase (NSE). The monoclonal antibodies used in the CanAg NSE EIA bind to the ?-subunit of the enzyme and thereby detects both the ?? and the a? forms (1, 2). The NSE levels are low in healthy subjects and subjects with benign diseases. Elevated levels are commonly found in patients with malignant tumours with neuroendocrine differentiation, especially small cell lung cancer (SCLC) (3) and neuroblastoma. Quantitative determination of NSE in serum may be valuable in the management of patients with suspected or diagnosed SCLC or neuroblastoma, to confirm the diagnosis, to monitor the effect of treatment and to aid in the detection of recurrent disease