The potential of bacteriophages as alternative treatment for multidrug-resistant (MDR) Klebsiella pneumoniae-related infections has recently gained much interest. The purpose of this research was to isolate and characterize a K. pneumoniae-specific lytic phage with the potential to treat experimental lobar pneumonia induced by K. pneumoniae in mice. A lytic phage was isolated from an urban wastewater sample in Tehran and characterized by transmission electron microscopy (TEM), thermal, pH, and chloroform stability before being employed for treatment of mice infected with K. pneumoniae in an experimental model of lobar pneumonia. BALB/C mice were challenged by intranasal inoculation with 10(8) colony-forming units (CFU/ml) of K. pneumoniae ATCC 10031 followed by an intraperitoneal injection of the isolated phage using 10(10) and 10(9) plaque-forming units (PFU/ml) simultaneously or 24 h post infection. Control groups of mice received bacteria or bacteriophage alone. Mice were euthanized daily up to 7 days post infection and examined for abnormality in their lungs and livers followed by determining the number of phages and bacteria in plasma and lung homogenates. The isolated phage (vB_KpnM-Teh.1) belonged to the Myoviridae family, was stable at 37 degrees C, pH 7, and was resistant to chloroform. Treatment of mice with a single dose of phage simultaneously at the time of infection, or 24 h post infection, resulted in seven and five logs decrease of CFU/ml in the lung homogenates up to 3 days after phage administration, respectively. The isolated phage may have the potential as a therapeutic agent against K. pneumoniae infections.