Netilmicin, a semisynthetic derivative of sisomicin, was tested against 327 isolates of Staphylococcus aureus, Pseudomonas aeruginosa and Gram-negative enteric bacilli. Seventy-two per cent of the isolates were inhibited at a concentration of 0.5 microgram netilmicin per ml, and 93% of the isolates were susceptible to 4 microgram per ml or less. The MICs of netilmicin and gentamicin for 24 Providencia and 38 Pseudomonas isolates were compared. The activity of netilmicin closely paralleled that of gentamicin, 46% of the Providencia isolates and 32% of the Pseudomonas isolates not being inhibited by 4 microgram per ml of either drug.