1. Venous plasma histamine and catecholamines were measured in stable asthmatic subjects by a recently developed specific and sensitive radioenzymatic assay.

2. Plasma histamine concentrations were significantly elevated in both extrinsic and intrinsic asthmatic subjects, compared with both normal controls and patients with chronic obstructive airways disease. There was no correlation between histamine concentration and severity of airways obstruction, however.

3. Elevated plasma histamine concentrations at rest-5-indicate increased release of mediators from ‘leaky’ mast cells in asthma.

4. Plasma catecholamine concentrations in asthmatic subjects did not differ from normal and there was no correlation with severity of broncho- constriction or with plasma histamine concentration.

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