Septicaemia was induced in anaesthetized and artificially ventilated rabbits. Three groups receiving different doses of Escherichia coli 055:B5 and a control group were studied. The animals were followed for 240 min after the start of the bacterial infusion. One animal in the high-dose and one in the medium-dose group died. Septic shock developed in the high-dose group. The infusion of E. coli led to a severe, sustained neutropenia and a dose-dependent thrombocytopenia. A severe arterial hypoxaemia developed during the infusion of bacteria in three of four animals, and in one of three animals in the high- and medium-dose groups, respectively. Both the levels of C5a in plasma and the accumulation of granulocytes in the lung tissue, as assessed by a morphometric method, were dependent on the dose of bacteria. A strong positive correlation was found between levels of C5a and endotoxin. The levels of both C5a and endotoxin correlated positively to the degree of accumulation of granulocytes in the lung tissue. The results from the present study suggest that the magnitude of bacteraemia and the degree of complement activation are probably important factors determining the course and the severity of septicaemia.