Human polymorphonuclear leucocyte (PMN) lysosomal cathepsin G exerts potent bactericidal action against Neisseria gonorrhoeae in vitro, independent of its serine esterase activity. The results presented demonstrate that (1) bactericidal, diisopropylfluorophosphate-treated cathepsin G binds in a specific and saturable manner to the surface of gonococci, (2) loss of carbohydrates in gonococcal LPS due to mutation increases total and specific binding of cathepsin G, and (3) at least three outer-membrane proteins (OMPs) (PIA, PIII, and a 45 kDa OMP) interact with cathepsin G. Taken together, the results suggest that gonococcal susceptibility to the lethal action of cathepsin G, and perhaps susceptibility of gonococci to oxygen-independent killing by PMNs, is controlled by LPS-masking of cathepsin-G-binding OMPs.