Sepsis caused by gram-negative bacteria is a common finding having high incidence and mortality. Fc alpha RI (CD89), a receptor for immunoglobulin A (IgA), has been shown to mediate bacterial phagocytosis, which might play a role in the pathogenesis of sepsis. In this study the expression and function of Fc alpha RI were analyzed on blood monocytes and neutrophils of patients with bacteremia. We found a marked increased in expression of the alpha- and gamma-subunits of the Fc alpha RI on both types of cells in patients with gram-negative bacteremia, but not in patients with gram-positive bacteremia. This increase was independent of serum IgA levels. Fc alpha RI M(r) was lower on cells from gram-negative patients than on cells from controls (50-65 kDa versus 55-75 kDa), despite a similar 32-kDa backbone, indicating altered glycosylation. Increased levels of Fc alpha RI on blood phagocytes correlated with enhanced serum IL-6 levels, but not with IFN gamma or TNF-alpha. FcR-gamma chain associated with Fc alpha RI was phosphorylated in patients neutrophils, indicating functional engagement of this receptor during gram-negative sepsis. Increased expression and activation of Fc alpha RI-gamma 2 complexes following gram-negative infections suggests its involvement in host defense against bacteria.