Fever in newborn lambs.

An initial injection of intravenous bacterial pyrogen in newborn lambs failed to produce fever, but did cause the typical fall in the number of circulating blood leucocytes. A second challenge of bacterial pyrogen in 60-h-old lambs caused fever. Other lambs also 60 h old, not previously injected with pyrogen, did not respond to a similar challenge. Thus, sensitization to bacterial pyrogen is necessary for development of fever in response to injection of bacterial pyrogen.