Klebsiella strains possessing capsule type K7 are found predominantly in respiratory secretions. To investigate the importance of this K antigen in virulence, 13 K7 strains were compared with K2 capsulate isolates which are generally regarded as highly virulent. The toxicity of the strains was determined in a mouse peritonitis model. Generally, K7 isolates were significantly less toxic for mice than K2 strains. In the absence of serum, neither capsule type showed much stimulation of leucocytes, measured as the chemiluminescence (CL) response of human polymorphonuclear leucocytes (PMNL). However, in the presence of normal human serum, CL values with K7 strains increased considerably, whereas the CL response to K2 isolates was unaffected. Correspondingly, intracellular killing by PMNL was observed with K7 strains only, whereas K2 isolates proved to be relatively resistant to phagocytic destruction. No correlation was found between capsule type K7 and serum resistance. These data suggest that, in contrast to K2, capsule type K7 may not be a critical factor in the virulence of K7-capsulate Klebsiella strains nor does it seem to act as an antiphagocytic barrier.