The basis of the histamine assay in skin.

1. The assay of wheal response to histamine injected sub-epidermally has been reviewed. Several discrepant claims were found in earlier work. 2. The saline control injection may evoke a wheal, which if present should be allowed for in the assay procedure. 3. There was no evidence for a limb dominance effect. 4. Using a 3.26 mM (1 mg ml-1) standard solution of histamine acid phosphate (1.18 mM as free base) and 0.1 ml injections, the straight central portion of the assay curve was found to be between 0.3 and 2.3 x 10(-3) mM, as free base, with an ED50 of about 0.018 mM histamine base.