Prevention of Klebsiella infections by passive immunotherapy has received more attention during the last decade. Both K antigen-and O antigen-specific antisera and monoclonal antibodies (mAbs) have been studied with respect to phagocytosis-enhancing and in vivo protective capacities. Our own work has focussed on the generation of O serogroup-specific rabbit antisera and O antigen specific murine antibodies. O-specific rabbit sera were absorbed extensively with heterologous O antigen strains in order to obtain highly specific typing reagents. Using these for typing a collection of 378 clinical strains, we found that 82% of them belonged to one of the four serogroups O1, O2ab, O3 and O5. Phagocytosis experiments using antisera and mAbs showed that O antigen specific antibodies were able to opsonize non-encapsulated strains, while fully encapsulated bacteria were rather resistant against the opsonizing effect. Nevertheless, in vivo experiments demonstrated a prophylactic effect on both Klebsiella septicemia and pneumonia in a mouse model of lethal infection. Given the limited number of O serogroups, O antigen-specific antibodies may be suited to supplement K antigen-specific hyperimmune globulins for passive immunoprophylaxis of Klebsiella infections.