An Oka strain varicella vaccine developed by SmithKline Beecham Biologicals in the early 1980s is registered for immunization of high-risk groups in nine European countries. Because the preparation must be stored at -20 degrees C, it was reformulated to facilitate its use for general vaccination in healthy children with storage at 2-8 degrees C for 2 years. Studies using production lots of the reformulated vaccine in approximately 1400 healthy children are summarized. During the 42-day follow-up, no vaccine-related serious adverse events were reported. Unsolicited reactogenicity rates were low: 14.2% in children ages 9-36 months (the main target age group for the vaccine). Seroconversion rates were 98.6% after a single dose. Consistent reactogenicity and immunogenicity were observed across vaccine lots. After efficacy is demonstrated in other studies, widespread use of this vaccine will prevent a common and potentially serious childhood illness.