The manganese (Mn) concentrations in serum of 40 healthy controls (9 males and 31 females), and of 26 patients with rheumatoid arthritis (4males and 22 females) were determined using a HITACHI MODEL 207 atomic absorption spectrophotometer. Wavelength of Mn determination was setted at 2795 A. Flow rate of acetylene was setted at 3.0 l/min and lamp current was setted at 10 rnA. The serum samples were ashed by IPC MODEL 1003plasma machine, and dissolved in 3N-HCl and evapolate on the hot plate, and redissolved in constant volume of 0.1 N-HCl. These sample solution were aspirated directly into the burner. Mn levels in serum were estimated by standard curve. The mean recoverry rate of ashedsample solution was 100.1%, and that of serum by ashing process was 101.6%. The serum Mn levels in 40 healthy controls and 26 patients with rheumatoid arthritis were 2.19 ± 0.34 (S.D.), 3.04 ± 1.01 (S.D.) μg/100ml respectively. The serum Mn levels in rheumatoid arthritis were significantly higher than healthy control levels (p<0.001). In 10 of 26 cases (38%) with rheumatoid arthritis, the serum Mn levels were shown to be above the upper limit of 5% rejection limit of healthy controls.