Eimeria tenella, aflatoxicosis, and a combination of the two prolonged the prothrombin times of male Cobb broilers. Factor V added to the plasmas of birds infected with E. tenella corrected the prothrombin time. E. tenella infection did not significantly alter the levels of plasma calcium, total plasma protein, or fibrinogen. Adding Factors V, VII, and X to plasma from birds with aflatoxicosis or aflatoxicosis plus E. tenella infection did not correct prothrombin times. Partial correction was achieved by adding normal plasma containing Factor II and fibrinogen. Plasma calcium and fibrinogen levels were not significantly altered in either group whereas total protein levels of these groups were significantly lower. The hemorrhage and mortality in birds infected with E. tenella may be explained in part by the longer prothrombin time resulting from deficiency of Factor V.