The macrolide antibiotics erythromycin and triacetyloleandomycin (troleandomycin) are prescribed for many types of infections. As such they are often added to other preexisting drug therapy. Thus, there are frequent opportunities for the interaction of these antibiotics with other drugs. Both erythromycin and triacetyloleandomycin appear to have the potential to inhibit drug metabolism in the liver and also drug metabolism by micro-organisms in the gut, either through their antibiotic effect or through complex formation and inactivation of microsomal drug oxidising enzymes. Of the two agents, triacetyloleandomycin appears to be the more potent inhibitor of microsomal drug metabolism. Published studies indicate that triacetyloleandomycin can significantly decrease the metabolism of methylprednisolone, theophylline and carbamazepine. Its ability to cause ergotism in patients receiving ergot alkaloids and cholestatic jaundice in patients on oral contraceptives may also be related to its inhibitory effect on drug metabolism. Erythromycin appears to be a much weaker inhibitor of drug metabolism. There are numerous reports describing apparent interactions of erythromycin with theophylline and a lesser number of reports dealing with carbamazepine, warfarin methylprednisolone and digoxin. There are sufficient data to suggest that erythromycin can, in some individuals, inhibit the elimination of methylprednisolone, theophylline, carbamazepine and warfarin. The mean change in drug clearance is about 20 to 25% in most cases, with some patients having a much larger change than others. Like tetracycline, erythromycin also appears to have the potential for increasing the bioavailability of digoxin in patients who excrete high amounts of reduced digoxin metabolites, apparently through destruction of the gut flora that form these compounds. Concurrent administration of triacetyloleandomycin with drugs whose metabolism is known to be affected or that could potentially be affected should be avoided unless appropriate adjustments in dosage are made. Coadministration of erythromycin with drugs believed to interact should be undertaken with caution and with appropriate patient monitoring. Among the other macrolide antibiotics, josamycin has seldom been involved in causing drug interactions, while midecamycin and the older derivative spiramycin have not so far been incriminated.