1. Erythrocyte carbonic anhydrase I was measured by immunoelectrophoresis in blood samples taken at intervals during the menstrual cycle of 26 normal women. Plasma progesterone concentrations were also measured in 18 of these subjects. In every case, the erythrocyte carbonic anhydrase I concentration reached a maximum at that point in the cycle when there was maximum secretion of progesterone. Statistical analysis indicated that the observed changes in erythrocyte carbonic anhydrase I concentration are highly significant and that erythrocyte carbonic anhydrase I and plasma progesterone concentrations are significantly correlated.

2. In three pregnant subjects the erythrocyte carbonic anhydrase I concentration showed a steady increase up to term.

3. Women taking progestogens in contraceptive pills, in doses of 1 mg/day or greater, had significantly higher concentrations of erythrocyte carbonic anhydrase I compared with females receiving no medication.

4. These observations strongly suggest that there may be a causal connection between plasma progesterone concentration and erythrocyte carbonic anhydrase I concentration in women.

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