By using flow cytometric analysis of cells in whole blood expressing high levels of CD14, we found a subpopulation of monocytes (8% of total) with higher scatter parameters, high capacity to produce reactive oxygen species (ROS), stronger expression of Lewis-X (CD15), sialyl-Lewis-X, CD11b and CD18 antigens, as well as an increased polymerized actin content. The size of this subpopulation increased after stimulation with lipopolysaccharide at the expense of the remaining monocytes, suggesting that its features were inducible. The membrane increase in Lewis-X and sialyl-Lewis-X expression observed during this conversion was largely due to the translocation of these carbohydrate structures from intracellular pools. Moreover, this subpopulation behaved as a primed monocyte subpopulation producing large amounts of H2O2 in response to N-formyl-methionyl-leucyl-phenylalanine. Increased H2O2 production was inhibited not only by anti-CD14 but also by anti-CD15 and anti-sialyl-Lewis-X monoclonal antibodies when added before lipopolysaccharide. These results show that lipopolysaccharide priming is regulated, at least in part, by Lewis-X and sialyl-Lewis-X structures expressed on the monocyte membrane. All together, this highly reactive and inducible subpopulation of monocytes, which share phenotypic and functional characteristics with neutrophils, might play an important role in host defenses and inflammatory responses.