Mercury(II) in aqueous solution can be determined by shaking with a dilute solution of silver-110m dibutyldithiocarbamate in chloroform, and counting the silver-110m extracted into the aqueous layer. Alternatively, the reagent in chloroform can be shaken with up to fifty times its volume of water, and the mercury present measured by the decrease in specific activity of the chloroform layer. The method is rapid, has a minimum sensitivity of 1 ng cm^–3 of mercury and a precision of ±5 per cent. Calibration graphs are linear, and the technique would be suitable for use in the field, in order to avoid losses of mercury during the storage or transportation of aqueous samples. Interference by chloride ions can be serious, but is completely overcome by adding potassium cyanide. Gold and palladium are the only metals that are likely to interfere.