Measurements of red cell counts, haemoglobin concentrations, packed cell volumes, reticulocyte percentages and pyruvate kinase levels were carried out daily in rabbits infected with either Trypanosoma brucei brucei S42 or T. b. brucei 427. With both stocks, rabbits became anaemic within 7 days, the anaemia being initially more severe in those animals infected with the S42 stock. Mean cell volumes, percentage of reticulocytes and pyruvate kinase levels increased during the infection, whereas mean cell haemoglobins remained relatively constant and mean cell haemoglobin concentrations decreased. Red cell T50 values were reduced in both infections, especially when the infecting organism was T. b. brucei S42. Bone marrow aspirates showed a general hyperplasia. The evidence is indicative of an anaemia which is haemolytic in origin, in spite of normal serum bilirubin concentrations. Possible mechanisms of haemolysis are discussed and it is concluded that the major cause is red cell damage due to the attachment to the erythrocytes of immune-complex factors, together with sequestration and destruction of red cells in the spleen and other organs of the reticuloendothelial system.