The minimal inhibitory concentrations (MICs) of iseganan (IB-367) ranged from 0.13 to 64 microgram/ml for gram-positive bacteria (Streptococcus mitis, Streptococcus sanguis, Streptococcus salivarius, and Staphylococcus aureus) and from 0.06 to 8 microgram/ml for gram-negative species (Klebsiella, Escherichia, and Pseudomonas). IB-367 exhibited rapid, microbicidal activity against both log- and stationary-phase cultures of methicillin-resistant Staphylococcus aureus and Pseudomonas aeruginosa. At concentrations near the MICs for these two organisms (4 and 2 microgram/ml, respectively), IB-367 reduced viability by more than 3 logs in less than 16 min. Similarly, IB-367 effected a 4-log reduction of the endogenous microflora in pooled human saliva within 2 min at 250 microgram/ml, a concentration readily attained by local delivery.