Thirty six Klebsiella pneumoniae strains were assayed in a test to determine bacterial interference; two of them had bacteriocinic effects on homologous species, strains 6 and 150 inhibited sensitive K. pneumoniae and they also acted on positive and negative Staphylococci coagulase. All cocci were sensitive to the bacteriocinogenic K. pneumoniae and none of the non-bacteriocinogenic strains inhibited them. Klebocin with homologous activity on K. pneumoniae seemed to be undistinguishable from the compound with heterologous action on Staphylococci in the aspects that were characterized in this work; both were heat labile to the same degree; optimum pH was 7, acidity decreased klebocin activity more intensely than alkaline pH. The antagonistic substance was not produced in the synthetic medium employed and was developed in tryptic soy, nutrient agar, brain heart agar and blood agar; tryptone-beef extract agar complex medium neither permitted the homologous activity nor allowed the interference on Staphylococci. The compound (or compounds) responsible for homologous antimicrobial effect had a low molecular weight as demonstrated by the fact that it pierces a dialysis membrane with molecular weight of 10,000 D cut-offs. Ethidium bromide treatment of strains 6 and 150 produced five strains without bacteriocinic activity which simultaneously lost their homologous and heterologous inhibitory capacity.