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African and American forms of malignant catarrhal fever (MCF) infections were produced by inoculating blood from affected cattle into susceptible cattle. Ease of disease transmission, incubation period, volume of blood required for infection, and clinical signs were compared in 30 cattle with African MCF and in 19 with American MCF. American MCF was more difficult to transmit than was African MCF and required eight to ten times more blood as inoculum. American MCF incubation period was more than twice as long as that of the African MCF, but the disease course was three times shorter. Clinical signs were similar for the two forms; however, in the American form, the disease was more acute and a larger percentage of animals had severe diarrhea.