The nature of the refractoriness of C6 glioblastoma cells to herpes simplex virus type I (HSV-I) infection has been studied. The cells were restricted in susceptibility to HSV-I since only a small proportion of the cells could be infected by HSV-I and the virus yield per cell was low. The susceptibility to infection was increased by treating the cells with trypsin-EDTA prior to infection. The cells so treated recovered resistance to the virus when incubated at 37 degrees C, their resistance being restored to the initial level in 2 days. This restoration was inhibited by addition of cycloheximide or puromycin. Trypsin-EDTA treatment of C6 cells increased the efficiency of adsorption of HSV-I and the formation of stable cell-virus complexes from which the virus could not be dissociated by heparin.