The A.A. have examined by Limulus endotoxin assay some LPS preparations from R. typhi, R. slovaka, C. burnetti phase I and II, as a demonstration of endotoxicity. All the preparations tested were able to gel the Limulus amoebocyte lysates, even if to a different degree of reactivity. The results add credibility to the hypothesis that LPS from Rickettsiaceae can represent at least in part the pathogenetic mediators of some manifestations described in Rickettsiosis; however, without endotoxin assay in the blood of patients, one cannot assume to this be true.