A microassay has been developed to measure histamine release from both washed human leucocytes and whole blood, in which blood samples can be diluted down to 1:20 without affecting their sensitivity to antigen. The sensitivity and reproducibility of the single isotope histamine radioenzymatic assay [REA] was compared to the conventional spectrofluorometric histamine assay. The limit of detection was 0.2 and 2.0 ng/ml, respectively, and the agreement between the two assays was good (r = 0.94). There was also a good qualitative agreement between skin-prick test and histamine release (92%). In order to see if this assay was suitable for small volumes of blood, we measured histamine release to antigen from both diluted whole blood taken by a thumb prick (as a model of the heel prick used in obtaining paediatric blood samples), and undiluted venous blood. There was complete agreement between a positive or negative histamine release response to antigen by diluted thumb prick and undiluted venous blood samples. Leucocyte sensitivity to antigen increased slightly upon dilution, but remained within a ten-fold antigen dilution. The micro-histamine release assay described here offers an opportunity to measure basophil sensitivity to allergens where the availability of the blood is limited (in young babies for example) as well as making possible multiple allergen testing.