We studied pyruvate kinase activity and its inhibition by alanine in 33 human brain tumours and nine normal adult blain tissues. With the exception of two schwannomas, there were no significant differences between the specific pyruvate kinase activities of tumours and of normal brain tissues. Residual pyruvate kinase activities of tumours were significantly lower than those of adult brain. The lowest residual activity was in metastatic adenocarcinomas (less than 15%). All patients suffering from metastatic adenocarcinomas died within 6 months after the operation. Astrocytomas of grade II and grade III showed relatively high residual pyruvate kinase activity. The reverse was found for glioblastomas. There was no correlation between the histological classification and residual pyruvate kinase activity in meningiomas. Our data suggest that alanine inhibition of pyruvate kinase can correlate with the histological grade of gliomas and postoperative survival of patients with brain tumours.