Experimental pneumonia was investigated by light microscopic, electron microscopic and morphometric methods in mice infected with the problem spore Klebsiella pneumoniae. In the early stage (2 to 7 hours post infection) we found Klebsiellae and aggregates of platelets in the pulmonary capillaries, but we could not establish significant structural alterations with morphometric methods. The ultrastructural investigation revealed alterations of the alveolar lining cells and the endothelial cells of capillaries and an infiltration of the interalveolar septula with polymorphonuclear and mononuclear leucocytes. These early changes were followed by an intraalveolar infiltration with inflammatory cells. Complement activation is considered as a trigger of the initial inflammatory reaction. The results of our study indicate that bacterial pneumonia starts in the interalveolar septula and secondary involves the alveolar spaces. A restitutio ad integrum of the lung develops 11 days post infection.