A variety of binding sites for endotoxin (LPS) have been identified on leukocytes. However, the sequence of expression of these receptors, and their interrelations, are poorly understood. In this report, we show that in LPS-responsive hosts, interaction of nanomolar concentrations of LPS with bone marrow cells induces the expression of new specific LPS-binding sites. Cells from LPS-nonresponsive (C3H/HeJ) mice do not express these receptors after LPS treatment. Experimental differences in the conditions allowing the induction and the detection of these binding sites (influence of Leishmania lipophosphoglycan, role of serum), support the hypothesis that interaction of LPS with primary receptors on bone marrow cells triggers the expression of secondary LPS receptors, and that the two types of receptors have distinct fine specificities.