Varicella-zoster virus (VZV) gene 62 encodes a protein with a predicted Mr of 140,000 (VZV 140K) that shares considerable amino acid homology with the immediate early (IE) regulatory protein Vmw175 of herpes simplex virus type 1 (HSV-1) and is believed to be its functional equivalent. We have tested this hypothesis by insertion of VZV gene 62 (expressed from the HSV-1 IE3 promoter) into both IE3 gene loci in the short region repeats of the HSV-1 genome. The parent virus used for this manipulation was D30EBA, which is a variant of HSV-1 from which the majority of the Vmw175 coding sequences have been deleted. Like other HSV-1 viruses lacking Vmw175 functions, D30EBA is able to grow only in cell lines which express Vmw175 constitutively. The resulting recombinant virus. HSV-140, is able to propagate (but unable to form obvious plaques) on normal cell lines. The properties of HSV-140 were studied by monitoring the time course of polypeptide expression and DNA replication during normal infection. We found that at high multiplicity HSV-140 synthesized apparently normal amounts of many viral polypeptides but that the expression of certain late genes was reduced; this slight defect may be related to less efficient DNA replication by HSV-140. At low multiplicity HSV-140 expressed viral proteins inefficiently. Surprisingly, VZV 140K was produced in large amounts at later times of a normal infection, indicating that the polypeptide fails to autoregulate the IE3 promoter. The results strongly suggest that VZV 140K is able to perform most of the functions of Vmw175 during growth of HSV-1, but that differences in detail lead to less efficient virus growth.