['Some researches have shown that melatonin (MLT) has effects on the erythrocyte deformability and on the osmotic fragility which, currently, seem to be heterogeneous and fragmentary.', 'This work wished to evaluate in vitro the possible modifying action of MLT at pharmaceutical doses on the osmotic fragility of rat blood samples treated with cytochalasin B (CB). The variation of the lactate dehydrogenase (LDH) levels have been measured because LDH is an enzyme which is considered an important marker of hemolysis. Working in a strongly reducing environment has been necessary in order to avoid possible antioxidant actions of MLT, measuring the plasma levels of malondialdehyde and total phospholipids in order to highlight possible MLT actions which are not related to its recognized antioxidant properties.', 'The data show a possible MLT action which strengthens the CB action on the osmotic fragility and on the membrane deformability of the erythrocytes.'] Does melatonin enhance the in vitro action of cytochalasin B on globular resistance and osmotic fragility of erythrocytes?