The action of some antibiotics on the terminal bile duct tone and motility was studied in vitro and in situ in various animal species (guinea pig, calf, cat, dog, monkey). Rifamycin SV, spiramycin, erythromycin, and some tetracyclines induce a constriction, while ampicillin, isoxazolyl penicillins, gentamicin, aminosidin, and chloramphenicol induce a relaxation of the choledochus. In vivo, the responses are independent of both duodenal activity and systemic effects related to the antibiotic flowing into the duodenum and absorbed by it. The antibiotics act directly on the biliary smooth muscle, probably by means of a specific interaction with the biophase. Factors influencing the action of antibiotics on the bile duct are: (a) structure-activity relationship, (b) binding into the bile, (c) pH of the bile, (d) time of contact, (e) time lag, (f) combination with other antibiotics. Some predictions in clinical pharmacology and therapeutics are discussed.