Nuclear factor-kappa B (NF-κB) is a multipotent transcription factor that plays a pivotal role in immune reactions, inflammation, and possibly hematopoiesis as well. Mobilization of neutrophilic granulocytes during inflammation is a highly regulated process, but one that is incompletely understood. We studied the in vivo activity of NF-κB in mouse organs and cells, with a focus on bone marrow, during acute inflammation. NF-κB activity was studied in transgenic mice expressing a luciferase reporter expressed in a NF-κB activation-dependent fashion. Acute peritoneal inflammation was induced by lipopolysaccharide (LPS), the casein digest bacto-tryptone, or the insoluble polysaccharide zymosan. Organs were removed and blood, bone marrow, and peritoneal cells were separated using density gradient centrifugation. NF-κB activity in organ homogenates and cell lysates was quantified. These three inflammatory agents increased NF-κB activity to a variable extent within the inflamed peritoneal cavity, liver, and spleen, with LPS being the strongest stimulus. LPS, but not bacto-tryptone or zymosan, activated NF-κB in lung and bone marrow, the latter activity mainly observed in density fractions rich in immature bone marrow cells. NF-κB activation was prominent at 6 h after induction of peritonitis, fading at 24 h, as expected for an acute phase phenomenon. From this proof-of-principle study with luciferase reporter mice dependent on NF-κB activation, we suggest that, in steady-state mice, mobilization of bone marrow granulocytes to an inflammatory site can occur without discernible activation of NF-κB in bone marrow.