1. The metabolism of (+)-, (-)- and (+/-)-salbutamol by sulphoconjugation was determined in vitro using human lung cytosol and bronchial epithelial BEAS-2B cell homogenate. 2. For the lungs the intrinsic clearance (Vmax/Km) value for the pharmacologically active (-)-salbutamol (0.49 +/- 0.32 ml min-1 g-1 protein) exceeded that of (+)-salbutamol (0.046 +/- 0.028 ml min-1 g-1 protein) by 11-fold. This was mainly due to a difference in Km value, which was 16 times higher for (+)-salbutamol (1300 +/- 170 microM) than for (-)-salbutamol (83 +/- 12 microM). 3. The stereoselectivity of sulphoconjugation of salbutamol was very similar in the BEAS-2B cells, although the absolute activity was considerably lower. 4. The enzyme catalyzing this reaction both in the lungs and in the BEAS-2B cells was the monoamine (M) form phenolsulphotransferase. 5. These observations emphasize that the smooth muscle of the bronchi most likely are exposed to considerably higher concentrations of the potentially toxic (+)-enantiomer than of the bronchodilating (-)-enantiomer during therapy with (+/-)-salbutamol.