Superoxide anion (O(-)(2)) plays an important role in the bactericidal activity of human monocytes. Stimulation of human monocytes by concanavalin A (Con A) and cytochalasin E (Cyt E) results in superoxide production. In this study, the O(-)(2) generation of monocytes was assessed for variation with age. Monocytes obtained from 51 normal subjects were assessed.The optimal concentrations of Cyt E and Con A were 20μg/ml and 100μg/ml, respectively. There was no significant difference in O(-)(2) production between males and females. O(-)(2) production in older subjects (65-86years) was significantly lower than that in younger subjects (20-64years)(p<0.05). The values were 11.2±3.3 nmol/min/10(6) cells in the former, and 12.9±2.6 nmol/min/10(6) cells in the latter. These findings suggest that impaired O(-)(2) production might be one of the factors accounting for the impaired bactericidal activity of mononuclear leucocytes in the aged.