Goats, sheep and calves were inoculated intravenously with strain Y3343 of the large colony type of Mycoplasma mycoides subsp. mycoides isolated from a goat with polyarthritis. The goats and sheep died of septicemia (one was killed in extremis) within eight days. The goats had leukopenia and granulocytopenia. Coagulopathy was indicated in some goats; the fibrinogen titer, prothrombin and partial thromboplastin times increased with the progress of disease and the number of platelets decreased dramatically in one goat. Goats and sheep had cellulitis at the site of inoculation, pleural hemorrhages, pneumonia, myocarditis, renal infarcts, glomerulitis, adrenal cortical necrosis, enteritis, focal splenic necrosis, polyarthritis and lymphadenitis. Vasculitis and thrombi were seen occasionally, suggesting that vascular changes, perhaps together with coagulopathy, had a role in pathogenesis. One of two experimental calves developed a slight fever, arthritis and minor inflammation of adrenal tissue. Calves seen less susceptible to the mycoplasma organism given intravenously than do goats or sheep.