Blood polyamine levels have been determined in 161 healthy subjects from newborn to adult age. During the growth period spermidine concentrations are always higher than in adulthood. In the first days of life a typical pattern for spermidine and spermine appeared with an early increase in the first hours after birth and a maximum at 24 h; afterwards the levels of both amines gradually and progressively decreased. The levels reached by 10 days of life were maintained until adulthood, at which time a further decrease was evident. The high levels of polyamines during the period of body growth may suggest that also in humans these substances play a role in the process of cellular proliferation.