We isolated a free-radical scavenger "water soluble protein (WSP)" from broad beans. Hydrocortisone (HC) is known to inhibit superoxide generation and was used as the reference scavenger. WSP was examined for its effect on antioxidation in young (PDL 20, 25% of the maximum life span) and old (PDL 50, 62.5% of the maximum life span) human fibroblasts (TIG-1). Cells were treated with WSP or HC for 4 and 6 wk in young cells, and for 3 and 6 wk in old cells. The cytosolic superoxide dismutase activity in the cells treated with WSP or HC tended to decrease as compared with that in the non-treated cells (control) with the exception of WSP-treated young cells 4 wk after culturing. Young cells were equal in glutathione peroxidase activity to the control, but the activity level in WSP- or HC-treated young cells 6 wk after culturing was 10-50% lower than that in the control. Young and old cells treated with WSP or HC were superior to the control in catalase activity with the exception of HC-treated old cells. WSP- or HC-treated cells were higher in glutathione (GSH) concentration than the control with the exception of WSP-treated young cells 4 wk after culturing and HC-treated old cells 6 wk after culturing. Such increases in catalase activity and GSH concentration by WSP treatment may be related to the delay of cellular aging-dependent degeneration.