We report the construction of a deletion mutant (del22Z) that is unable to synthesize any detectable messenger RNA or protein products from the herpes simplex virus type 1 (HSV-1) immediate early ICP22 gene upon infection. The del22Z deletion mutant lacks all but 18 nucleotides of the ICP22 coding sequence and carries the bacterial lacZ gene at the site of the deletion. No other known open reading frames or flanking sequences were disrupted. Del22Z was able to infect Vero cells productively but was severely restricted in human and rodent cells that were permissive for the parental HSV-1(F). The yield of del22Z was not enhanced significantly, either by increasing the multiplicity of infection or by increasing the duration of the infection. There was a prolonged expression of some early gene products and a delayed appearance of some late gene products in both permissive and restrictive cells. This phenotype of cell-line restricted growth and alteration of the normal gene expression cascade maps specifically to the ICP22 coding region.