In previously published studies, we employed a photoreactive radioiodinated derivative of LPS from Escherichia coli 0111:B4 to identify and characterize a membrane-localized specific LPS binding protein of approximately 80-kDa molecular mass. Our more recent studies demonstrating that mAb with specificity for this 80-kDa protein will act as an agonist in mediating macrophage activation have established that this protein serves as a specific receptor for LPS. In the experiments reported here, we have more accurately determined the apparent molecular mass of this protein to be 73 kDa (p73). We have also extended the sources of LPS-derivatized photo-cross-linking preparations (including Re-LPS) to determine generality of LPS binding to this receptor. Binding to the p73 LPS receptor is demonstrated with all of the LPS derivatives synthesized in our laboratory, as well as probes synthesized by other investigators. Binding of S-LPS is readily inhibited by Re chemotype LPS, and we have shown that this competitive inhibition is most likely not the result of formation of LPS aggregates. These results confirm and extend our earlier studies suggesting that the binding of LPS to the p73 receptor is lipid A specific. We further demonstrate that, in contrast to results published in a recent report, the p73 LPS receptor has no significant binding specificity for a variety peptidoglycan polymer preparations. Finally, we show that this LPS receptor can be detected on murine fibroblast, macrophage, and mastocytoma cell lines. Differences have been observed in the level of expression of LPS receptors on the various cell lines studied.