histamine release tests using whole blood were applied in testing in vitro Type I birch allergic reaction in two patients with allergic rhinitis. Heparinized blood was incubated with varying dilutions of allergen for 30 min at +37 degrees C. After centrifugation of the blood, plasma was separated and the liberated histamine was analysed by histamine radio enzyme assay (REA), high performance liquid chromatography (HPLC) with post-column derivatization system or radio immuno assay (RIA), and the results obtained by these methods were compared. REA and HPLC gave similar results, while RIA gave somewhat higher values, but was less sensitive.