The method is intended for the estimation of minute quantities of histamine in blood plasma. A neutralized trichloracetic acid extract of plasma is adsorbed at pH 8 on a column prepared by mixing a quantity of the cationic exchange resin Amberlite XE-64 with powdered cellulose as a supporting medium. Histamine is adsorbed but N-acetylhistamine is not: the amines can therefore be estimated independently. Elution of the histamine is by displacement with HCL. The eluate is converted into the solution for bioassay on the superfused guinea-pig ileum.In recovery experiments histamine was added in the range 25 to 100 ng. to solutions of known composition and to plasma (5 ml.) obtained from man and the cat. The mean recovery in this range for histamine added to plasma was 82.5% A2 S.E. (17 estimations).The histamine equivalent of human plasma obtained from the antecubital vein was found to be less than 1 ng./ml.It was possible by this method to follow the concentration of histamine in the arterial plasma of the cat when histamine was infused intravenously at a rate of 330 ng./kg./min. There was no perceptible change in the arterial blood pressure during the infusion, but the plasma histamine rose from <0.3 ng./ml. to 3.4 ng./ml. (mean of 2 estimations).When the quantity of histamine extractable from plasma is less than can be estimated by conventional methods of bioassay, the sensitivity of the test can be increased by applying the undiluted extract directly to the strip of guinea-pig ileum; this may be suspended in a minute bath (Mongar and Schild, 1950) or in air, as in the technique of superfusion (Gaddum, 1953). The method is applicable if the histamine has been sufficiently purified and if the test solution is similar in composition to the solution bathing the intestine between the doses.If plasma is extracted by the method of Barsoum and Gaddum (1935) as modified by Code (1937a) and tested in this way, the activity of the extract is only partially abolished by the histamine antagonist mepyramine maleate (Mongar and Whelan, 1953). Moreover, the method does not distinguish between free histamine and histamine which may be released from pharmacologically inactive forms at the stage of refluxing the extract in strong acid (unpublished results).Various chromatographic methods of purifying histamine have been proposed and are reviewed in the article by Code and McIntire (1956). Some employ paper chromatography, others weak cationic exchangers which are used to remove histamine from solutions containing organic solvents.These methods are designed mainly for the chemical determination of histamine, but some have been combined with bioassay.In the method to be described the purification is conducted entirely in aqueous solution with the aid of a carboxylic ion exchange resin, at room temperature and at pH values not far from neutral. Advantage is taken of the fact that carboxylic resins have a high exchange capacity and can be buffered over a wide range of pH. They can therefore be used to separate weak organic bases in a...