1. Induction of tolerance of (CBA X A) hybrid skin homografts by the injection of (CBA X A) hybrid spleen cells on the 4th postnatal day could usually be prevented by pretreating the A strain recipients either on the day of birth or on the second day with 200,000 hybrid spleen cells. 2. This effect could not be detected when 200,000 cells were administered on the day of birth and the tolerance-conferring dose was injected on the second day. 3. It is concluded that mice can be effectively sensitised within 4 days of birth, and this essentially confirms the observations made by Howard and Michie (6) with a different test system; our apparent failure to sensitise within the first 2 days of life may well have been due to the insensitivity of our test system. 4. It is argued that these results, as well as the data of Michie and Howard, do not make it obligatory to abandon the stem cell theory of induction of tolerance.
Received April 1, 1963.
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