It is generally accepted that the allergic reaction depends on the levels of specific IgE, on the concentration of blocking antibodies and on the capacity of the basophils to release mediators. The fact that in the histamine release test (HRT) all of the elements intervening in an allergic reaction (antigens, IgE, basophils and mononuclear cells of a lymphoid origin) have been found, has led some authors to consider it as the most suitable method of "in vitro" diagnosis. This is a reflection of what biologically occurs "in vivo". Furthermore, the HRT can be used as a control parameter following immunotherapy, permitting us not only to quantify the level of blocking antibodies but also to study the possible presence of a control mechanism of histamine release. Accordingly, in a group of 50 patients with extrinsic asthma we carried out a HRT, skin tests (prick test) and Rast with house dust, Dermatophagoides pteronyssinus and Dermatophagoides farinae. In another group comprising 74 subjects with extrinsic allergic asthma (45 with perennial allergic rhinitis) we performed a HRT with aeroallergens. This group was subjected to immunotherapy (dust with mites) for a year. Following this treatment another HRT was carried out with house dust, Dermatophagoides pteronyssinus and Dermatophagoides farinae. With regard to our results in the first group we obtained 80% agreement in the HRT/prick test/Rast for house dust and Dermatophagoides pteronyssinus and 78% agreement for Dermatophagoides farinae.(ABSTRACT TRUNCATED AT 250 WORDS)