The in vitro efficacy of netilmicin and its synergism with penicillins were examined. Netilmicin was effective against E. coli, indole-negative and -positive Proteus, Klebsiella, Enterobacter, Salmonella, Shigella and Serratia. Netilmicin was less effective than other aminoglycosides against Citrobacter and Pseudomonas. A combination of netilmicin and piperacillin synergistically inhibited Enterococcus. Netilmicin was not synergistic with either penicillin or carbenicillin against Enterococci. Netilmicin with either carbenicillin, ticarcillin or piperacillin synergistically inhibited Pseudomonas aeruginosa, but no such synergism was noted against highly resistant strains of Pseudomonas.