Antistreptolysin O (ASO) levels vary with age group of the study population and geographical locations. The present study was undertaken to determine the upper limit of normal of ASO in 200 normal children of 5-15 years of age with no history of recent sore throat infection. The standard tube dilution method (WHO) was used for estimating ASO titers. It was found that 239 IU was the upper limit of normal in the study population, which can be considered as the baseline ASO titer. This can provide useful guidelines for physicians in the interpretation of elevated ASO titers in cases of suspected acute rheumatic fever.