The neutrophil counts of seven women, three tkaing oral contraceptives and four nottaking them, showed cyclical variations during the menstrual cycle, most consistentlya fall in the neutrophil count at menstuation. The neutrophil count in women not taking oral contraceptives rose to a peak twice during each cycle. One womennot taking oral contraceptives was studied in detail over eight consecutive menstrualcycles. She showed two neutrophil peaks per cycle and a similiar variation in themonocyte count. The eosinophil count showed a reciprocal relation with the neutrophil count and the basophil count fell in mid-cycle. The changes in her neutrophil count seemed to follow changes in oestrogen level with a delay of one to two days. Oestrogen probably promotes release of neutrophils from the bone marrow rather than from the marginated pool.