Glioblastoma multiforme (GBM) is caused by the central nervous system-derived glial cells, and represents the most common (50%-60%) form of primary brain tumors. The aim of this study was to investigate the in vitro effects of selenium on human GBM cells. In the present study, GMS-10 and DBTRG-05MG human GBM cell lines were used as a model to examine selenium entering the cell, cell proliferation, cytotoxicity, DNA fragmentation and Ki-67 protein expression in selenomethionine treated and non-treated groups. Seleno-L-methionine (SeMet) as the organic source of selenium exerted effects on cell proliferation and cytotoxicity, as assessed with WST-1 and lactate dehydrogenase (LDH) tests, respectively. Apoptosis was assessed by DNA fragmentation with an enzyme-linked immunosorbent assay. Ki-67 protein expression was determined by Western blotting, while selenium measurements were performed in the supernatants and lysates by using Graphite Furnace Atomic Absorption Spectrometry. This is the first study to examine the effects of SeMet on cell proliferation and death in GMS-10 and DBTRG-05MG cells. Both GBM cell lines responded to SeMet in a dose- and time-dependent manner. WST-1 test showed that low-dose SeMet treatment (50 and 100 mu M) increased cell proliferation. Analysis of intracellular SeMet levels by using AAS showed results consistent with viability and cytotoxicity tests. SeMet treatment for 72 h caused increased DNA fragmentation in both cell lines. In conclusion, our results suggest that SeMet induces cell death at high doses, while increasing cell proliferation at low doses. In the view of the data obtained in this investigation, further studies focusing on the possibility of using SeMet against different types of GBM and in combination with prospect synergic compounds are considered to be worthwhile.