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.