Phosphonoacetic acid inhibited replication of simian varicella virus (Delta herpesvirus) in tissue culture. The drug was tested in patas monkeys 40 h after infection with Delta herpesvirus. A total of 200 mg/kg per day was given intramuscularly, divided into two doses every day for a total of 10 days. The treated monkeys were protected from clinical illness, and Delta herpesvirus was not recovered from their lymphocytes. Complement-fixing and neutralizing antibody titers were significantly lower in phosphonoacetic acid-treated monkeys than in the untreated controls. In animals given the drug alone, there was dermatitis and blackening of the skin and hair, serum glutamic oxalacetic transaminase and serum glutamic pyruvic transaminase enzymes were significantly increased, and liver biopsy revealed diffuse cytoplasmic swelling and granulation of the hepatocytes. The therapeutic range of this drug should be studied carefully before considering its use in severe varicella-zoster infection in humans.