AbstractPrevious findings have shown that lipopolysaccharide (LPS)-activated human monocytes express cytokines (CKs) on their membrane. Furthermore, those associated to membrane products such as tumor necrosis factor (TNF)-αand interleukin (IL)-1 have been demonstrated to exert many biological activities. In this paper, evidence is provided that human polymorphonuclear cells (PMN) exhibited an increased phagocytic capacity following incubation with either lipid A (LA)-activated autologous monocytes or supernatants recovered from LA-stimulated mononuclear cell cultures. In order to investigate the possible role of monocyte membrane-associated TNF-α, IL-1αand IL-1βin the modulation of PMN activity, in a separate series of experiments LA-activated monocytes or LA-activated supernatants were pretreated with anti-recombinant human (Rhu) TNFα, anti-Rhu IL-1αand anti-Rhu IL-1βmonoclonal antibodies (MoAbs), respectively. Such an approach gave rise to an abrogation of monocyte-mediated triggering effect on PMN functional capacity.Taken together, these data suggest that activated monocytes can upregulate PMN phagocytosis by a cell-to-cell contact mechanism, likely related to membrane-associated CKs.