Pacidamycins are nucleosidyl-peptide antibiotics which have activity only against Pseudomonas aeruginosa. Their MICs for other organisms such as Enterobacteriaceae, Staphylococcus aureus, most Streptococci and other Pseudomonas species are >100μg/ml. These compounds had no activity against erythromycin-susceptible Streptococci. The MICs for Streptococcus pyogenes with constitutive- and inducible-type of macrolide-lincosamidestreptogramin resistance were 12.5 and 25 μg/ml, respectively. The MICs against P. aeruginosa ranged from 8 to 64 μg/ml. The activity of these compounds was 1 to 2-fold less in serum than broth. Time-kill curves were performed using 4 and 8 times the MIC of pacidamycin 1. It was bactericidal against P. aeruginosa (3 Iog₁₀ decrease in 4 to 6 hours). At 24 hours, resistant mutants were found in the cultures. The MICs of piperacillin and gentamicin for these mutants were the same as for the parent strain. The frequency of resistance to these compounds was <3.5×10^-6. The resistant mutants were stable after 10 transfers in antibiotic-free medium. The pacidamycins were inactive against P. aeruginosa in mouse protection tests. After a single subcutaneous injection of 25 mg/kg of pacidamycin 1, the Cmax was approximately 50 μg/ml and the serum half-life was 0.5 hour.