In two groups of patients with well-defined obstructive lung disease – a group of bronchitics according to WHO criteria and a group of patients with extrinsic allergic asthma – the effect of carazolol, a so-called non-cardioselective β -blocking drug, was investigated. Pindolol was used to compare the results with a second blocking drug. Carazolol increased bronchial resistance in the bronchitic group but not in the asthmatic group; with the exception of 1 patient in whom carazolol induced a status asthmaticus. The effect of the drug on bronchial resistance depends only in part upon the drug itself, but was influenced much more by the characteristics of the patients. We therefore conclude that the quality of β₁-receptorspecificity was, at least in part, a result of the qualities of the patients. The designation of a drug as β₁-specific should therefore be used very cautiously. Serious side-effects on the bronchial system in particular persons are also possible from drugs shown to be safe in some groups of patients