We have studied the influence of protamine on the sensitivity of three strains of Pseudomonas aeruginosa to various antibiotics: streptomycin, gentamicin, polymyxin B, and beta-lactams. While protamine enhanced the antibacterial action of beta-lactams towards P. aeruginosa, it did not alter the effect of aminoglycosides. The antibacterial power of polymyxin B, on the other hand, was drastically reduced. The observed changes in this strain's sensitivity to antibiotics could be due to changes in the permeability of the outer membrane.