DNA was extracted from nucleocapsids isolated from WI-38 cells infected with two different strains of varicella-zoster virus (VZV). The DNAs were treated with each of six restriction endonucleases (EcoRI, HindIII, Bg/I, Bg/II, Sal I and Bam-HI) and small, but reproducible differences in restriction endonuclease patterns were observed. These strains were passaged in WI-38 cells and in primary guinea-pig embryo (GPE) cells, followed by restriction endonuclease analysis of the DNAs. No changes were observed in the restriction prpofile of the DNA of one of the strains (VZV-KMcC) after 46 passages in WI-38 cells but small differences were observed after 72 passages. No changes were observed after 30 passages of another strain (VZV-AW) in WI-38 cells. Twenty passages of VZB-KMcC in GPE cells did result in minor alterations of its DNA. It was concluded that VZV DNA was sufficiently stable after multiple passages in WI-38 cells to make restriction endonuclease analysis a valuable epidemiological tool for strain differentiation.