Levels of factor IX:C and factor IX:Ag were measured in 120 healthy subjects with an age range of 1 to 67 years. Levels of factor IX:C were lowest in prepubertal subjects, then reached a plateau in early adult life with a secondary increase in later adult life (> 45 years). Changes in factor IX:Ag showed a similar trend. Factor IX:Ag disproportionately increased in early adult life, however, with a less pronounced increase in later life, resulting in peaking of the ratio in early adult life. It is suggested that caution be exercised in interpreting low normal factor IX:C levels in prepubertal children, and that interpretation of ratios of antigenic to coagulant activity may need to take subject age into consideration.