1. An initial injection of intravenous bacterial pyrogen in newborn lambs failed to produce fever, but did cause a fall in the number of circulating blood leucocytes.

2. A second challenge of bacterial pyrogen in 60-h-old lambs caused fever, whereas other 60-h-old lambs, not previously injected with pyrogen, were unresponsive.

3. Thus the newborn lamb requires sensitization, by exposure to endotoxin, before pyrogen-induced fever can occur.

4. When bacterial pyrogen was injected intravenously into the unanaesthetized foetus in utero, foetal and maternal body temperatures remained unchanged in response to the initial challenge and to one or two subsequent injections of bacterial pyrogen in utero, or in the lamb immediately after birth. In all instances the foetal white cell count fell in the 90 min after the injection.

5. The response to both bacterial pyrogen and leucocyte pyrogen appears to depend upon the maturity of the physiological mechanisms involved in pyrogen fever.

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