Acivicin (AT-125) is a glutamine antagonist with dose-limiting, schedule-dependent CNS toxicity and predictable CSF penetration after intravenous administration. Because of these properties, a trial in CNS malignancies was initiated. Thirty-two patients with recurrent or residual malignant astrocytomas were treated with AT-125. The majority of patients had glioblastoma multiforme (24) and had received prior nitrosoureas (21). The median age was 50 years, and Southwest Oncology Group (SWOG) performance status was 2. The major determinant of response was based upon radiologic criteria using computed tomographic (CT) scanning and/or magnetic resonance imaging (MRI) scans. The tumor mass was measured in two perpendicular planes, which yielded the largest cross-sectional area. Standard solid tumor criteria for response were used. All responding patients also had a stable or tapered dose of corticosteroids with stable or improved performance status and neurologic examination. There were four objective responses (12%): one complete remission (3 1/2+ years) and three partial remissions (57, 86, and 322 days). Two patients had improvement in disease that did not meet requirements for a partial remission. Toxicity was mild and primarily consisted of nausea, vomiting, and lethargy. Two patients were removed from study due to neurotoxicity (depression and hallucinations). The strict response criteria used in this trial were not those that have been used in testing other active agents such as carmustine (BCNU). We conclude that AT-125 has objective antitumor activity in malignant astrocytomas and warrants further study.