Calves infected with Trypanosoma congolense TREU 112 had generalized microvascular dilation which was most prominent in the liver and the mesentery; there also was increase in cellularity in the lung with significant alveolar thickening and accumulation of hemosiderin-bearing cells in the alveolar septa, focal accumulations of lymphocytes primarily at the corticomedullary junction of the kidneys and a well developed glomerulonephritis, likely membranoproliferative. Reticuloendothelial changes were marrow hyperplasia with an erythroid shift, moderate hemosiderosis and moderate dysthrombopoiesis, marked thymic cortical atrophy, hypersplenism and enlarged nodes with reduced cellular density, paracortical atrophy and medullary sclerosis.