This study showed differences in plasma and erythrocyte copper concentrations in children and adults. All determinations were performed with a flameless atomic absorption spectrophotometer. The mean plasma copper level at 1 month of age was significantly lower than in adults. After 1 month of age the mean concentration in plasma increased to its peak value at 2 to 5 yr of age, then decreased gradually with age. At 7 months to 10 yr of age, the copper levels in plasma were significantly higher than in adults. The erythrocyte copper levels at 1 month to 1 yr of age were significantly higher than in adults. The copper content of erythrocytes was highest at 2 to 6 months of age and then decreased gradually. The copper concentration of erythrocytes may reflect more accurately liver and total body copper levels than does the plasma copper level. There is less possibility of copper contamination in erythrocytes than in hair. Therefore, the measurement of erythrocyte copper concentration may well be a helpful index of the total copper status.