Clinical observations indicate that beta-adrenergic drugs may increase bronchial reactivity in asthmatics. To find out possible reasons for this phenomenon the beta-adrenergic receptor function of isolated lymphocytes of asthmatic patients treated with clenbuterol alone or with ketotifen and clenbuterol together were studied. The cAMP levels of lymphocytes stimulated by different doses of isoproterenol were measured by radioimmunoassay and have been compared in the groups of healthies, and asthmatic patients after 3-months running of clenbuterol (Spiropent, Sandoz), as well as in the same asthmatics after one-week running of parallel administration of ketotifen and clenbuterol. There was no difference between the beta-adrenergic receptor function in asthmatic patients treated with clenbuterol alone vs. untreated healthies. Applying ketotifen and clenbuterol together the beta-adrenergic receptor function increased compared to the values obtained after application of clenbuterol alone (intraindividual-control) as well as vs. the group of healthies (control). Data presented support the view that therapeutic doses of selective beta 2-agonists do not lead to damage of the beta-adrenoceptor function. The improvement of receptor function after parallel administration of clenbuterol and ketotifen may be a consequence of the participation of ketotifen in the control of beta-adrenergic receptor system. Thus it seems unlikely that down-regulation of beta-adrenergic receptors is responsible for the beta-agonist induced bronchial hyperreactivity. That's why TXB-2 levels in the plasma of the same asthmatic patients and healthy volunteers were determined by RIA.(ABSTRACT TRUNCATED AT 250 WORDS)