The first live-attenuated Oka strain varicella vaccines needed to be stored at -20 degrees C. Reformulation of this vaccine by SmithKline Beecham Biologicals has provided a vaccine shelf life of up to 2 years when stored at +4 degrees C to +8 degrees C. In this study the immunogenicity and reactogenicity of two different production lots of this reformulated vaccine at two different titres each, which corresponded to the release and expected expiry titres, were evaluated. A double-blind randomised clinical trial was conducted in healthy children aged from 9 to 24 months. Immunogenicity was assessed by the measurement of varicella specific antibodies in paired serum samples taken before and from 35 to 63 days post vaccination. Reactogenicity was assessed by the evaluation of any untoward reactions occurring up to 42 days post vaccination. In order to assess protective efficacy, parents of these subjects were contacted approximately 6 months after completion of the trial. One hundred and ninety-one subjects were recruited into the study. Of the 181 initially seronegative subjects who completed the trial according to the protocol, 179 showed seroconversion (98.9%). Reactions to the vaccine were minor and observed in 46/191 (24%) of subjects. Rashes were present in 19, fever in 22, and both fever and rash in 5. Rashes were mainly maculo-papular in nature but were vesicular in 6. Febrile reactions were shortlived. After a 6-month follow up period, attenuated varicella with minor clinical symptoms was diagnosed in 6/52 vaccinees who had close contact with natural varicella (attack rate = 11.5%). CONCLUSIONS This reformulated vaccine was well tolerated, highly immunogenic and provided protection against varicella. In increased stability allowing refrigerator storage makes it a good candidate for mass vaccination programmes.