SUMMARY A purification step involving separation of the ketonic fraction through the Girard complex has been incorporated in an earlier method for estimating urinary oestrogens. The new method, which measures oestrone only, involves acid or enzymic hydrolysis of the urine, extraction with ether, removal of the acidic fraction with alkaline carbonate solution, evaporation of the ether, separation of the ketonic fraction through the Girard complex, saponification, methylation, and chromatography on alumina columns. The oestrone methyl ether present in the final extract is measured by a semi-micro modification of the Kober reaction, which, with the larger volume of urine processed, increases the sensitivity of the estimation ninefold. This increase in sensitivity is made possible by the purification achieved in the new method, and fractions of 1 μg oestrone/24 hr urine can be measured. The reliability of the new method has been investigated and data concerning its accuracy, precision, sensitivity and specificity are presented. By comparison with the new method, the earlier method gives overestimates of approx. 0.9 μg oestrone/24 hr urine.