BACKGROUND: Antistreptolysin O (ASO) is very useful as an indicator of recent streptococcal infections and their sequelae, such as rheumatic fever and acute glomerulonephritis. To interpret single ASO level of patients, the upper limit of normal (ULN) ASO from the same age group in the area should be known. As Streptococcus pyogenes infections are quite common in elementary school, we measured ASO and analyzed them by the results of throat culture to determine upper limit of normal ASO of school children in Chinju area. METHODS: ASO concentrations were measured quantitatively by nephelometry on 436 sera of healthy elementary school children in Chinju area. Throat cultures were taken at the same time to evaluate the relationship between ASO concentrations and throat culture results, including serogroup, colony forming units (CFU), and M types. RESULTS: The mean ASO concentration was 285IU/ml and the upper limit of normal ASO was 433IU/ml. The ASO levels were even (253-285IU/ml) through whole school grades except the 5th grade (350IU/ml). Not only the carriers of group A streptococci, but also those of group C or group G streptococci had higher ASO levels. The children from whom more than 10 CFU of S. pyogenes were isolated showed higher ASO levels than those who had less than 10 CFU. The ASO levels were higher in M type 6 or 22 compared to M type 12 or 28. CONCLUSIONS: The upper limit of normal ASO of children in Chinju was 433IU/ml, that is between Seoul(326IU/ml) and Chungnam (499IU/ml). The children who had more than 10 CFU tended to have higher ASO levels, which indicate asymptomatic infections, are associated with burden of bacteria. Group C or group G streptococci may induce serum response like group A streptococci. Certain M types may be implicated as strong producer of streptolysin O.