Detergent must be added to the strongly acidic Kober reagent to obtain reproducible and well separated peaks in continuous flow analytical methods for oestriol in pregnancy urine. Brij 35 and other polyether detergents are not satisfactory because they or their impurities interfere in the reaction causing high blank response in colorimetric reactions and diminished fluorescence responses. In a single phase fluorometric method these detergents also caused turbidity or phase separation in the solution which passed through the fluorometer. Some anionic sulphate and sulphonate detergents were also unsatisfactory being immiscible with Kober reagent. Selected cationic and betaine detergents are shown to be possible alternatives, although one which is satisfactory in the kober reagent may be unsuitable for a dilution reagent. The betaine Empigen BB was found to be suitable for inclusion in both of these reagents.