Viruses originally isolated from the blood of patients with typical signs of measles and which exhibit similar cytopathogenic and antigenic properties have been propagated serially in cultures of human renal cells (1). Inoculation of the cultured virus of the first, second, and twenty-third tissue-culture passages into cynomolgus monkeys that did not originally possess circulating antibodies capable of reacting with the virus was followed by one or more manifestations of an infection resembling measles in man. When all were present, these manifestations consisted of a definite period of incubation, viremia, exanthem, leukopenia and the development of specific complement-fixing and virus-neutralizing antibodies. Virus recovered from the blood of monkeys during the acute phase could not be distinguished from the virus that was inoculated.

Complement-fixing and virus-neutralizing antibodies capable of reacting with the cultured virus have been demonstrated in the sera of a large majority of rhesus and cynomolgus monkeys that had been held in captivity for some time. Such antibodies were not found in the sera of 31 cynomolgus monkeys shortly after capture. Spontaneous infection of 5 cynomolgus monkeys with an agent that gave rise to antibodies reacting specifically with the cultured virus was observed while the animals were in our possession.

On the basis of these findings and of others previously reported, it is concluded that:

  1. Characteristic agents isolated from the blood and throat washings of human patients with measles and shown to be capable of unlimited multiplication in cultures of human cells are members of the viral species responsible for this disease.

  2. Cynomolgus and rhesus monkeys frequently incur spontaneous infections with this virus or a closely related agent.

  3. Only monkeys without serologic evidence of such infection may be expected to develop signs of measles after experimental inoculation of the virus.

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