The objective of this study is to investigate (a) whether heavy—ion (56Fe) radiation exposure activates one of the key transcriptional regulators, nuclear factor kappa B (NF-kB), in normal human monocytes (Mono Mac 6 cells) and (b) whether the induction of NF-kB by high LET radiation mediates the down-stream gene expression such as pro-and/or anti-inflammatory cytokines. The Mono Mac-6 cells were exposed to an iron (56Fe) beam at a dose rate of 11 Gy/min. The doses were 0.2, 0.35, 0.7 and 1.4 Gy. For the downstream gene expression study the cells were exposed to a total dose of 0.5 Gy. The induction of DNA-binding activity in the nuclear extracts was analyzed at 2, 4, 8 and 24 h by the electrophoretic mobility shift assay, using [g-32P] end-labeled NF-kB specific oligonucleotide probes. The study revealed that the activation of NF-kB after the heavy-ion exposure was both dose and time dependent. A profound increase in the DNA binding activity was observed at 2hrs post exposure. At the 2hrs time point, the response after the 0.7 Gy dose was the maximum for all doses and times studied. After the maximum at 2hrs post exposure, for all but the 0.2 Gy dose, the DNA binding activity in the nuclear extract decreased with increasing time post-exposure. These results clearly indicate that (a) heavy-ion radiation can induce NF-kB DNA binding activity in normal human monocytes; (b) the activation is rapid, and could precede downstream NF-kB target gene expression; and (c) the heavy-ion induced activation is persistent. Since activation of NF-kB by extracellular stimuli is implicated in inflammation, infection and cancer induction, as well as in protection of cells against insult, it will be important in subsequent studies to elucidate whether heavy-ion induced NF-kB activation is involved in high LET-specific phenotypic expression. We acknowledge the beam time provided by Dr. Gregory A. Nelson, Loma Linda University Radiobiology Program (supported by NASA Cooperative Research Agreement NCC9-79).