Netilmicin is a new, semisynthetic aminoglycoside antibiotic active against some gentamicin-resistant gram-negative bacteria. In this study we compared a 10-,ig netilmicin disk with the standardized 10-jig gentamicin disk in terms of their abilities to predict probable clinical susceptibility to netilmicin. The Netilmicin (Sch 20569) is a new, semisynthetic aminoglycoside derived by ethylation of the 1-N position of the deoxystreptamine ring of sisomicin (9). It has been shown to have in vitro activity similar to that of sisomicin and gentamicin against gram-negative pathogens (4,5,7,8) and also to be active against some gentamicinresistant strains of gram-negative bacteria (6-8). Netilmicin also has been shown to have a pharmacokinetic profile similar to that of gentamicin (8). This in vitro study compared a 10-,ug netilmicin disk with the standardized 10-,Ig gentamicin disk in terms of their abilities to predict probable clinical susceptibility or resistance to netilmicin.One hundred eighteen clinical isolates of gram-negative bacilli were studied. These included 25

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