The present study evaluates the effect of the naturally occurring dipeptide carnosine on primary cell cultures established from patients with glioblastoma multiforme. Surgically removed tumors were used to establish primary cell cultures that were incubated for 96 h with medium supplemented with carnosine at concentrations of 20, 40 and 50 mM. Following incubation, dehydrogenase activity, cellular adenosine triphosphate concentration (ATP), caspase activity, lactate dehydrogenase (LDH) release and the rate of DNA synthesis were determined. After 96 h of carnosine treatment a significant reduction in cellular ATP and dehydrogenase activity was detected already at a concentration of 20 mM carnosine. Carnosine (50 mM) reduced ATP concentration to 42.7 ± 13.5% (n = 6) and dehydrogenase activity to 41.0 ± 19.3% (n = 6) compared to untreated cells. Additional experiments revealed no sign of enhanced apoptosis or necrosis in the presence of carnosine. However, a quantitative bromo-desoxy-uridine-based proliferation assay demonstrated a clear effect of carnosine on DNA synthesis reducing its rate down to 50% (2 cultures) and 10% (4 cultures). Therefore, it can be concluded that carnosine is obviously able to inhibit proliferation of cells derived from glioblastoma. Since it is a naturally occurring substance that appears to be non-toxic to normal tissue and is able to penetrate the blood-brain barrier it may be a candidate for a therapeutic agent that may reduce proliferation of neoplastic cells even in vivo and especially in cases of glioblastoma multiforme.