We investigated the potential mechanisms of tamoxifen cytotoxicity in the U-373, U-138, and U-87 human glioblastoma cell lines, namely interference with protein kinase C (PKC) activity, the oestrogen receptor, and/or the production of transforming growth factor β1 (TGF-β1). We further examined the effects of tamoxifen on the cytotoxicity exerted by γ-radiation, 1,3-bis(2-chloroethyl)-1-nitrosourea (BCNU), and etoposide in this cell line panel. Thus, the cells were treated for 4 days with tamoxifen, γ-radiation, purified recombinant human TGF-β1 (rhTGF-β1), BCNU, or etoposide, either alone or at certain combinations. Cellular responses were evaluated with the sulphorhodamine B assay, as well as by multiple drug effect analysis, and related to PKC activities in particulate and cellular fractions; cellular oestrogen receptor contents; and the influence of rhTGF-β1 on cell growth. Tamoxifen inhibited cell proliferation as well as the phosphorylation capacity of the particulate, but not of the cytosolic fractions dose-dependently, at comparable kinetics, and at IC 50 values of approximately 15 μM. At these concentrations, tamoxifen acted synergistically with γ-radiation (4- to 6-fold) and additively with BCNU (approximately 2-fold), but did not affect etoposide cytotoxicity. The cells were negative to immunostaining for the oestrogen receptor, and rhRGF-β1 did not influence their growth up to 100 nm. Our data suggest that tamoxifen can sensitise cultured glioblastoma cells not to etoposide but to γ-radiation and BCNU, possibly through interference with membrane PKC, supporting its evaluation in experimental protocols for primary malignant gliomas.