Two lots [RS 3/2 and RU 4/17] of Wistar RA 27/3 Rubella virus vaccine with about 50-fold difference in concentration of virus per dose were administered to 203 children of less than 5 years of age with negative immunization or natural rubella history and followed for 6-7 weeks. Complete information was collected on 194 children. Eighteen of the 19 children with positive prevaccination hemagglutination inhibition [HAI] titers responded with 4-fold or higher rise following vaccination with no side reactions. The sero-conversion rate with both lots, among children with negative prevaccination HAI titer varied from 80-90%, a rate lower than reported by other [90-100%], the rate being higher with RS 3/2 lot which had lower virus concentrations per dose as compared to RU 4/17 lot. Furthermore, the percentage of vaccines reporting side reactions [26-52%] was much higher as compared to the earlier reports [10-34%], the side reaction rate being higher with RU 4/17 lot than RS 3/2 lot. The reactions were mild and of short duration and required no hospital care. There was no arthralgia or lympho-adenopathy reported. In this study the higher sero-conversion and lower side reaction rates were associated with RS 3/2 vaccine low which had a lower virus titer than RU 4/17 vaccine lot.