Summary Pneumococcal infection in mice was studied by following physiological changes and the course of the bacteriaemia during the infection. Significant increases in the activity of plasma glutamate oxalacetate transaminase (GOT) and glutamate pyruvate transaminase (GPT) were recorded, along with a rise in potassium and a fall in blood glucose. There was no significant change in plasma alkaline phosphate, inorganic phosphorus or sodium. Similar changes in GOT and GPT were recorded after injection of a cell-free pneumococcal product. These changes were compared with those occurring in other septicaemias. Antibiotic therapy dramatically reduced the bacterial numbers without preventing death. Such results suggest that the pneumococcus produces a toxin or toxins in vivo.