The absolute bioavailability of prednisolone was assessed in ten asthmatic patients who responded poorly to ordinary doses of corticosteroids so-called "steroid-resistant" bronchial asthma. Plasma levels of prednisolone were measured by a high pressure liquid chromatographic method after oral (20 mg) and intravenous (18.6 mg) single-dose administration. Determination of the number of eosinophils in the blood was used as an estimate of the effect of the drug. The total plasma clearance, plasma half-life and volume of distribution determined from intravenous data were similar to those reported earlier for healthy volunteers and asthmatic patients. Orally administered prednisolone was rapidly absorbed and found to have complete bioavailability. There was a similar decrease in the number of eosinophils in the blood both after oral and after intravenous administration. Thus, the present study does not support the hypothesis that resistance of asthmatic symptoms to oral corticosteroids is due in some cases to poor absorption or rapid elimination of these drugs.