Ultrastructural evidence for differentiation in a human glioblastoma cell line treated with inhibitors of eicosanoid metabolism. Human glioblastoma cells incubated in the presence of inhibitors of eicosanoid biosynthesis show decreased cellular proliferation without cytotoxicity. We studied the ultrastructural morphology of a human glioblastoma cell line cultured with nordihydroguaiaretic acid (NDGA), a lipoxygenase inhibitor, or 5,8,11,14-eicosatetraynoic acid, a cyclooxygenase and lipoxygenase inhibitor. When glioblastoma cells were treated for 3 days with antiproliferative concentrations of either agent, they shared many morphological characteristics, including evidence for increased astrocytic differentiation with only limited signs of toxicity. The inhibited glioma cells demonstrated an increase in the number and length of astrocytic processes containing greater numbers of glial filaments, and the NDGA-treated cells also demonstrated extensive lateral pseudopod formation along the processes. The glioblastoma cell shape also became more elongated, losing the usual nuclear lobularity and nuclear inclusions, especially in NDGA-treated cells. Many cytoplasmic organelles packed the cytosol of the inhibited glioma cells, including prominent Golgi apparatus, dilated smooth endoplasmic reticulum evolving into dilated vesicles, cytoplasmic vacuoles, and numerous concentric laminations. There was limited evidence for toxicity, however, as the mitochondria were more pleomorphic with some mitochondrial distention and disruption of the cristae along with an increase in cytoplasmic vacuolization. We conclude that the inhibitors of eicosanoid biosynthesis, NDGA and 5,8,11,14-eicosatetraynoic acid, not only suppress glioblastoma cell proliferation, but also induce increased astrocytic differentiation. 