Endotoxin (lipopolysaccharide, LPS) has the property of inducing tolerance to its own biological effects. This phenomenon has been extensively studied in animal models but only few studies exist on the regulation in humans. Here we describe experiments designed to determine the cytokine regulation and cellular changes in humans during induction of LPS tolerance after repeated LPS injections. Intravenous administration of purified LPS Salmonella abortus equi to cancer patients induces high amounts of circulating tumor necrosis factor-alpha (TNF-alpha), interleukin-6 (IL-6), interleukin-8 (IL-8), granulocyte colony-stimulating factor (G-CSF), and macrophage colony-stimulating factor (M-CSF). Repeated injections of LPS at daily intervals resulted in a marked downregulation of the cytokine response and in the case of TNF-alpha, IL-8, G-CSF, and M-CSF the cytokine response was reduced to baseline levels. In contrast, significant increases in serum IL-6 were detected up to day 5 of repeated LPS injections. Hematological changes included transient decreases in WBCs affecting granulocytes, monocytes, and lymphocytes, followed by a marked granulocytosis. The drop in WBCs remained unaltered throughout the 5 day course of repeated LPS injections whereas the granulocyte overshoot recovery diminished gradually. When PBMCs of the cancer patients were restimulated ex vivo a marked enhancement of the capacity to produce TNF-alpha, IL-113, and IL-6 occurred, which is in contrast to the decreasing TNF-alpha serum levels obtained in vivo. In parallel, a shift in monocyte subpopulations from CD14+/CD16- to CD14+/CD16+ cells was observed. The data provide evidence that different mechanisms are implicated in the cytokine downregulation following repeated LPS injections to cancer patients. Furthermore, PBMCs from LPS tolerant patients do not demonstrate a reduction in their capacity to produce cytokines.