Although many epidemiological studies indicate protective effect of vitamin C against a variety of human malignancies its mechanism(s) of action is questionable. The presented results show that the part of its effect may be accomplished by mononuclear cells, as necessary participants in body defence. Namely, in a long-term in vitro assay we tested vitamin C influence on random migration ability of malignant pleural effusion mononuclears (PEM) obtained from breast cancer patients. Vitamin C in a dose- (50-500 micrograms) and time-dependent (4-44 h) manner inhibited PEM motility, suggesting that immobilization of cells in situ may contribute to its beneficial effect in human cancers.