The in vivo effect of ascorbic acid on human natural killer (NK) cell activity was determined. Twenty control healthy subjects were given ascorbic acid at a concentration of 60 mg/kg, and blood was drawn at 0,1,2,4,8, 24 and 48 hours after treatment with ascorbic acid. Peripheral Blood Lymphocyte-NK activity was measured by a 4-hr.-51Cr-release assay using K562 tumor cells as targets. Treatment with ascorbic acid was shown to have a biphasic effect on NK activity: a transient slight suppression between 1 to 2 hrs. (20% of control) was followed by a significant enhancement (an over-shoot) at 8 hrs. that was further increased at 24 hrs., then the activity returned to the normal level by 48 hrs. Changes in the activity of ascorbate treated NK cells were inversely related to the E:T ratio; namely 231%, 189%, 141% and 127% at 6:1, 12:1, 25:1 and 50:1 E:T ratio respectively. Flow cytometry analysis indicated no quantitative changes in the NK cell sub-populations post treatment with ascorbic acid in the experimental subjects as compared with control untreated subjects. Simultaneous to measurement of NK count and activity, ascorbic acid and its uptake by PBL was measured in the plasma. The uptake of the vitamin was maximized at 2–4 hours and maintained at a high level up to 24 hours. We conclude that ascorbic acid is a potent immunomodulator and its effect in enhancement of NK cytotoxicity may explain one mechanism by which ascorbic acid exerts its probable anti cancer activity.