Early endotoxin tolerance has been defined as the transient period after an initial sublethal exposure to LPS during which a normally responsive individual is rendered hyporesponsive. Little is known about the cellular mechanisms that underlie this phenomenon. In this study, an early tolerance system was established by the injection of mice with 25 micrograms of E. coli K235 LPS. Maximal hyporesponsiveness in response to a challenge injection was observed 3 to 4 days after the initial injection, and normal responsiveness returned by 8 days after the initial exposure to LPS. Further experiments described herein demonstrate that the acquisition and maintenance of the tolerant state coincides temporally with an increase in the number of macrophage progenitor cells in the bone marrow. Cell-sizing profiles of the bone marrow cells from tolerized mice indicate an enrichment for a population of cells that are significantly larger than in bone marrow preparations from control mice. By density gradient sedimentation, it was shown that the denser population of cells from tolerized mice contained the increased numbers of progenitor cells, which, by cytology, were immature monocytic cell types. The increased numbers of macrophage progenitors was sustained after a second (challenge) injection of LPS. These results indicate that early endotoxin tolerance is associated with an increase in a population of bone marrow cells that is enriched for macrophage progenitors and suggests the possibility that the lack of responsiveness observed during the hyporesponsive period is related to a failure of these immature cell types to respond to LPS.