This study showed differences in erythrocyte manganese concentrations in different age groups and in cord blood. All determinations were performed with a flameless atomic absorption spectrophotometer. The mean erythrocyte manganese concentration at one month of age was 435.1 +/- 118.7 ng/g Hb, three to four times higher than in adults. Thereafter, it decreased rapidly and was constant from 4 months to 11 yr of age. However, at 12 to 19 yr of age the concentration in males decreased (108.1 +/- 20.2 ng/g Hb) and was significantly lower than in females (141.7 +/- 25.5 ng/g Hb) (p less than 0.01). This sex difference became even greater in adults aged 20 to 40 yr, when it was 91.5 +/- 22.4 ng/g Hb in males and 141.2 +/- 19.8 ng/g Hb in females (p less than 0.001). The erythrocyte manganese concentration in adults was 31.6 +/- 9.2 ng/ml, somewhat higher than the value previously reported, which may reflect dietary habits in Japan or a racial difference.