Herpesvirus type 1 could be propagated most efficiently in cultured fetal neurons, to a lesser extent in NG108-15 neurohybridoma cells, and with the lowest titer in glial cells. Herpesvirus type 2 could not be cultured in neurohybridoma cells, and in fetal neurons a titer 100-fold lower than for herpesvirus type 1 was obtained. Cells infected with herpesvirus type 1 were used in an infectivity assay for acyclovir dose-response studies. The ED50 values were 7.4 nmol/l for fetal neurons, 180 for neurohybridoma cells, and 275 nmol/l for glial cells.