1. Pregnant guinea pigs were given a subcutaneous injection of CNS tissue in water-in-oil emulsion containing killed mycobacteria. Their offspring delivered at the time of the onset of encephalomyelitis in the mothers had no lesions in the CNS.

  2. Rats were given multiple, simultaneous, intracutaneous injections of an encephalitogenic emulsion and 7 or more days later joined in parabiosis with normal rats as recipients. The couples were sacrificed after 7 to 12 days of parabiosis. Twenty pairs were killed 15 days after the injection of the donors and 8 days of parabiosis. Histologic examination of the CNS of the 40 rats revealed encephalomyelitis in 12 donors and 5 recipients. These 5 recipients which had lesions were mates of the positive donors.

  3. Control experiments showed that 9 days after active sensitization only 1 of 22 rats had encephalitis.

  4. The reasons for the view that passive rather than active sensitization occurred in the parabiotic recipients are discussed.

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