Rabbits made tolerant by several injections of OT lost their ability to produce detectable amounts of endogenous pyrogen (EP) in response to the specific agent (OT) but mobilized normal amounts of EP when given a small unrelated stimulus. On the other hand, when this stimulus followed shortly after an initial tuberculin fever of sufficient magnitude, release of additional EP was suppressed, presumably due to an inhibitory effect of the EP previously mobilized by tuberculin.

Similarly, a single large dose of endotoxin almost completely suppressed the response of sensitized rabbits to OT given several hours later. Since several spaced injections of the same dosage were ineffective, this phenomenon does not appear to be attributable to the known mechanisms by which endotoxins promote non-specific resistance to toxicity and infection. Tolerance to tuberculin could not be definitely shown following an injection of Newcastle disease virus which also produces a circulating EP, and it has been inferred that endotoxin blocks the pyrogenic action of antigen on host tissues directly rather than through mobilizing EP.

On the basis of these observations, the relationship of specific to non-specific tolerance to tuberculin fever has been compared in terms of the ability of such tolerant animals to mobilize EP to heterologous stimuli and it is concluded that the two forms of tolerance are different. Furthermore, the fact that a number of unrelated agents produce tolerance non-specifically supports the concept that there may be a common source of EP released by a number of stimuli, including endotoxins and myxoviruses, as well as antigen in specifically sensitized hosts.