The antimicrobial action of polycation proteins from the nuclei--histons with a high content of lysine (f1) and arginine (f3)--on Pseudomonas bacteria was studied. The sensitivity of various species of the same genus, and various strains of the same species, to histons was differential. The differences do not depend on the ability of the cells to liberate into a surrounding medium substances inactivating histons, and on the rate of histon sorption on the cells. Interaction between the bacteria and histons interferes with the permeability of the membranes, so that components of a low molecular weight, including those with the maximum of absorption in UV at 260 nm, are liberated into the surrounding medium. The total RNA of the cells is depolymerized and the activity of RNase increases. No correlation was established between these phenomena and the sensitivity of the bacteria to histons. The duration of the lag period was also studied, as well as the growth rate in the exponential phase and the total yield of the cells; a positive correlation was detected between the duration of the lag period and the sensitivity of the bacteria to histons.