In a series of 60 patients 62 intraoperative measurements with the 32-P (radiophosphorus) tumour marker were performed. Using miniature semiconductor probes a reliable discrimination between normal brain and neoplastic tissue was possible in nearly all brain tumours. The best results were found in meningiomas, where even small, visually hardly discernible tumour resides within the matrix zone could be reliably detected. Only in low-grade gliomas the application of the 32-P marker was impossible due to count rates similar to or below the basic rates of normal brain. This simple to use, non-invasive method proved its usefulness in all situations where a local radical tumour removal was important.