The effects of oral practolol (Eraldin, I.C.I. Ltd) (300 mg) and acebutolol (Sectral, May & Baker Ltd) (300 mg) were compared in a placebo-controlled, crossover study in ten asthmatics. 2. Pulse rate, forced expiratory volume in one second (FEV1) and specific airways conductance were measured before and after a bronchodilator and the same procedure was repeated after taking acebutolol, practolol or placebo. 3. A mean increase in resting airways obstruction and reduction in bronchodilator response were evident with both drugs. The differences were statistically significant only after acebutolol for the logarithm of specific conductance. Mean plasma levels of the two drugs were similar. The response to both drugs was similar in the same patient and unrelated to severity of asthma. Mean reduction in resting pulse rate was statisitcally greater after acebutolol compared with placebo but not after practolol compared with placebo. 4. In any individual the beta-adrenoceptor blocking response is unpredictable and any beta-adrenoceptor blocking drug must be used with extreme caution in asthmatics.