Nineteen assessable patients with recurrent malignant astrocytomas who had failed standard therapy (surgery, radiation, and/or chemotherapy) were treated on a phase I-II trial with a biologic extract of Serratia marcescens (ImuVert; Cell Technology, Boulder, CO) a new biologic response modifier (BRM). Two complete responses (CRs) were seen, of 63 and 77+ weeks duration. One minor response (MR) occurred, of 6 weeks duration. There were four additional stable (S) patients, with durations of 58+, 39, 12, and 7 weeks. Median time to progression and median survival in the CR plus MR patients were 63 and 129+ weeks, respectively. Overall, median time to progression and median survival were 12 and 19 weeks, respectively. Three patients are alive greater than or equal to 2.5 years from study entry. Common toxicities included transient (less than 72 hours) tenderness, induration, and erythema at the injection sites. Systemic toxicities were less frequent and included fever, chills, nausea/vomiting, headache, arthralgia, and hypotension. The response rate (CR plus MR) to this new BRM is modest (16%). However, the observation of CRs in patients with advanced recurrent malignant astrocytomas, with acceptable overall toxicity, warrants further study of this agent.