A wildebeest calf was reared in captivity and infected with malignant catarrhal fever (M.C.F.) virus by the intravenous inoculation of virulent cattle blood. It developed a viraemia, detectable in calf thyroid cultures, from the 8th day to the 31st week post-inoculation; no virns was detected in the blood by 37 cultural tests during the 32nd to 85th weeks p.i. The maximum titre attained was about 102·0 T.C.D. 50 per ml. of blood and there was no free virus in the plasma. From the 2nd to the 12th weeks of the experiment, viraemia was continuously present and 3 bovine calves which were housed with the wildebeest acquired the infection, apparently by contagion and after incubation periods of 30, 38 and 45 days. From the 13th week onwards the viraemia became intermittent and difficult to detect; bovine calves which were maintained in close contact with the wildebeest during this time were not infected. in a second experiment, a wildebeest calf which was captured when 6 months old was also inoculated with virulent cattle blood but only an intermittent, low-level viraemia was detected in the following 8 weeks. No virus was detected in 40 further cultural tests, extending to the end of the 15th month p.i. Bovine calves did not acquire M.C.F. by close contact with this wildebeest during the first 8 weeks of the experiment. During 1961 many wildebeest calves were abandoned by their dams, due to a severe drought in a National Park. Viraemia tests on 22 of these animals yielded a single strain of M.C.F. virus, derived from a calf less than 1-week old. Efforts were made to rear 9 calves and these were subjected to regular weekly tests for viraemia; with 1 exception all acquired the virus by the 15th week of life, presumably by contagion. In 3 individuals the duration of viraemia was shown to be at least 3, 12 and 36 weeks. Two bovine calves, exposed to contact with the wildebeest during the early stages of their viraemia, acquired M.C.F. after maximum incubation periods of 47 and 81 days. Further cattle, exposed later to the wildebeest, did not become infected. These observations and those of the preceding paper are discussed with reference to the epizootiology of M.C.F. in cattle in East Africa.