Anti-streptolysin O (ASO) antibodies are an expression of the frequent encounters even during the first years of life with beta hemolytic Streptococci, they are easily measured by quantitative laboratory tests and so possess the characteristics of a screening test for impaired antibody production. We have assessed ASO titers in 1955 healthy subjects of different ages (range: 1 month to 97 years). The behaviour of ASO titer is extensively described in the text. The percentage of people with less than 10 TU titers is under 5% after the age of 5 years up to 15 years; from 15 to 60 years there are no subjects with undetectable ASO titer and after this age the percentage is still under 5%. It seems therefore advisable not to define a "normal" treshold when titrating these antibodies to avoid the risk of missing low antibody producers.