Using whole blood, we examined the effects of stress hormones on CD69 expression in Natural Killer (NK) cells. A series of diluted adrenaline or cortisone was added to 1 ml of whole blood. Propranolol was used to block the beta-adrenergic effect. The blood samples were activated with CD2/CD2R mitogenic antibody solution for four hours under 37 degrees C. Then CD69 antigen on NK cells was analyzed by flowcytometric assay. Adrenaline and cortisone doses dependently suppressed CD69 expression on NK cells. Propranolol blocked the suppressive effect of adrenaline. CD69 expression was significantly higher than the control when only propranolol was added. We concluded that blood constituents acting on mitogenic reaction and stress hormones affect the early stage on the way to proliferation and differentiation.