Leukemia inhibitory factor (LIF) exhibits multiple biological activities in various tissues, and we have shown that LIF activates POMC gene transcription in response to immune signals. As higher serum levels of LIF have been reported in septicemia, we measured LIF values in biological fluids by RIA. Immunoreactive LIF was detected in 303 of 428 human serum samples. Circulating LIF detection rates were 69% in acute inflammatory diseases, 83% in chronic inflammatory diseases, 61% in noninflammatory diseases, and 90% in cancer patients. Serum concentrations of human LIF was higher in patients with inflammatory disease than in noninflammatory disease (0.80 +/- 0.10 vs. 0.53 +/- 0.02 ng/mL; P < 0.05) or in cancer patients (0.44 +/- 0.06; P < 0.05). Higher serum human LIF levels were found in septicemia (0.78 +/- 0.14 ng/mL), pneumonia (0.80 +/- 0.10 ng/mL), acute bronchitis (0.88 +/- 0.09 ng/mL), other infections (1.01 +/- 0.17 ng/mL), and systemic lupus erythematosus (SLE; 0.79 +/- 0.06 ng/mL). In 7 septicemia patients, Gram-negative infection was associated with higher LIF levels (1.06 +/- 0.16 ng/mL) than was Gram-positive infection (0.58 +/- 0.14 ng/mL). In patients with acute inflammatory disease, serum LIF levels decreased within several days after hospitalization. To test circulating mouse (m) LIF changes in response to inflammatory stress, lipopolysaccharide (LPS) was injected ip to mice. LPS increased serum mLIF values concordantly with ACTH levels. After i.p. injection of 80 microg LPS, serum mLIF increased by 144% (P < 0.05), 173% (P < 0.05), and 134% at 30, 90, and 120 min respectively. In vitro, however, LPS did not increase ACTH and mLIF secretion from dispersed mouse primary pituitary cells. These results suggest that LIF is an important participant in the pathogenesis of the acute inflammatory response. The elevated serum LIF levels observed in inflammation do not appear to originate from the pituitary.