In inflammatory and infectious diseases, the presence of circulating cytokines in plasma strongly suggests, following their exacerbated production, that saturation of specific binding sites has occurred or that an equilibrium between receptor-bound and free cytokines has been reached. In this report, we demonstrate that in addition to circulating interleukin-8 (IL-8), high levels of cell-associated IL-8 were detected in blood samples from patients with sepsis syndrome. The following analysis will reveal that in addition to erythrocytes, which have been dubbed a "sink" for IL-8, peripheral blood mononuclear cells (PBMC) and polymorphonuclear cells (PMN) contributed to the detection of cell-associated IL-8. On a per cell basis, 2,000 to 7,000 times the amount of IL-8 was found associated with PMN than with erythrocytes. In addition, circulating cells may well be the source of the leukocyte-associated form of IL-8. Similarly, in vitro experiments, such as whole-blood stimulation assays or the addition of exogenous IL-8 in blood samples, demonstrated that a large proportion of the IL-8 was associated with leukocytes. This suggests that the trapping of free cytokines onto the cell surface and the internalization of the IL-8 bound to its receptor, occurring both in vitro and in vivo, allows the detection of this cell-associated form. This analysis of cell-associated cytokines was extended to IL-1ra, another component of the inflammatory response, which, in contrast to IL-8, has been demonstrated to exist as an intracellular form. Indeed, cell-associated IL-1ra was also detected in septic patients. The measurement of cell-associated proinflammatory and anti-inflammatory cytokines in patients is clearly a more reliable reflection of their production than is the simple measurement in plasma and may provide useful indication to further understand the inflammatory process.