Variation in natural killer activity in peripheral blood during the menstrual cycle.

Natural killer activity was measured sequentially in normal female volunteers through their menstrual cycle. During the periovulatory period there was a significant fall in natural killer activity compared with in normal male volunteers. This variation was not apparent in women taking oral contraception. Cytotoxic activity was not related to oestradiol concentrations in individual women. The data support an interaction between immunological activity and sex hormones over the normal physiological range and would account for the described reduction in natural killer activity in pooled blood from female blood donors.