During an outbreak of rubella, more than 4,000 primary schoolchildren who had been vaccinated with BCG before entrance were tested with tuberculin. According to the status of rubella infection, they were grouped into 3 groups: those having experienced the disease long ago, those having shown clinical manifestations of rubella during this epidemic, and those without any evidence of past or recent disease. There was a significant depression of mean reaction size for the second group compared with those in the other two. The breakdown of the second group into 4 subgroups according to the time interval between the tuberculin test and the time of onset of the disease made it clear that the reaction was smallest within 20 days after onset and that it was then gradually restored to the healthy level after 30 days. There was no evidence of suppression of hypersensitivity among subclinical infection cases. Methodologic problems concerning tuberculin data analysis in this kind of research are discussed, with an emphasis on the importance of a quantitative approach to these considerations.