In order to elucidate how virulence is controlled in encapsulated bacteria, some surface properties of an encapsulated but avirulent strain of Klebsiella pneumoniae, strain 277, were examined. Although strain 277 was heavily fimbriated, the fimbriae did not demonstrate an avirulent character and were not responsible for the surface hydrophobicity of this strain. The surface hydrophobicity was well correlated with the capacity of the bacteria to associate with polymorphonuclear cells. More bacteria with hydrophobic surfaces associated with the PMN than nonhydrophobic bacteria. The hydrophobic surface character of this strain was not affected by either trypsin treatment or extraction with salt solution. We assume that the capsule of strain 277 has more hydrophobic polysaccharides than that of the virulent strain. Some chemical modifications might therefore exist in the capsular polysaccharides of the avirulent strain.