Studies in vitro , in dogs and in man have shown that aminoglycosides are inactivated by penicillins. In vitro the rate of inactivation depends on the concentration of the penicillin. Penicillin G, ampicillin, carbenicillin, ticarcillin, mecillinam, azlocillin, mezocillin, and piperacillin all have been shown to inactivate aminoglycosides, but amikacin is less susceptible to inactivation than kanamycin, gentamicin or tobramycin, with netilmicin intermediate. In dogs the same interaction has been demonstrated between carbenicillin and aminoglycosides. The serum half-lives of gentamicin and tobramycin are reduced in the presence of a constant high level of carbenicillin but that of amikacin is little changed. Similar inactivation continues in refrigerated samples. In man, the effect of carbenicillin on gentamicin, but not on amikacin was confirmed in patients with end-stage renal failure. The half-life of gentamicin was 61·6 h in the absence of carbenicillin, and 18·4 h in its presence. Because of its stability in the presence of penicillins, amikacin appears to be the drug of choice for the treatment of severely ill patients. Trois etudes sont presentées Ce fait est particulièrement important quand la pénicilline et l'aminoglycoside sont en contact prolongé, tel que, chez les malades ayant une fonction rénale altérée. D'autres rechercheurs ont observe ce phénoméne chez des malades àfonction rénale normale. L'effet de cette inactivation des aminoglycosides doit être pris en compte dans le traitement des malades immunodéprimés. Puisque l'amikacine est l'aminoglycoside le moins inactivé par les pénicillines elle devrait ètre l'aminoglycoside de première intention chez ce type de malades.