Undifferentiated U937 cells are non-permissive for herpes simplex virus (HSV) infection but can be rendered permissive by treatment with phorbol myristate acetate (PMA), which causes them to differentiate to a macrophage-like phenotype. Following infection with HSV, both PMA--treated and untreated cells correctly transcribe the viral immediate-early genes at levels comparable to those observed in fully permissive cell types, but immediate-early RNA and protein are detected only in the PMA-treated cells. Hence PMA acts by relieving an early block to HSV infection caused by the rapid turnover of immediate-early RNA. This block is not caused by the production of soluble inhibitors and can also be relieved by treatment with other agents that cause macrophage differentiation such as 1, 25 dihydroxycholecalciferol. These findings therefore indicate that the non-permissivity of undifferentiated U937 cells for HSV is mediated by post-transcriptional regulation of immediate-early gene expression.