Restriction endonucleases which cleave DNA at specific nucleotide sequences can be used to produce a set of DNA fragments of a viral genome which, when separated by gel electrophoresis, gives a characteristic "fingerprint" for that virus genome. This simple technique has been used to identify and classify DNA viruses of the herpes, adeno, and papova virus groups. Small variants within a given type (e.g., herpes simplex type I) are genetically stable and permit study and identification of individual strains of viruses. Such analyses have recently been applied to study the epidemiology of some DNA virus outbreaks. Restriction endonuclease fingerprinting provides a useful addition to methods for virus identification and classification.