Because of ongoing terrorist activity and dissemination of nuclear materials, the possibility of military or civilian personnel being exposed to ionizing radiation is a continuing threat. As yet, no drugs have been approved by the US Food and Drug Administration for treatment of hematopoietic or GI injury from penetrating ionizing radiation. Our research program aims to identify and develop pharmacological radiation countermeasures to prevent, mitigate, and treat the acute radiation syndrome. In light of the logistical realities of likely nuclear disaster scenarios, much of our current focus is on drug candidates with extremely low toxicity and ease of administration, suitable for use outside the clinic without physician supervision. Several promising radiation countermeasures are currently in various stages of development. Among the efficacious drugs, CBLB502 (truncated flagellin, a toll-like receptor-5 agonist and NF-kB activator), the steroid 5-AED (5-androstenediol), CBLB612 and CBLB613 (synthetic lipopeptides triggering activation of NF-kB through TLR2 receptor complexes), and tocopherol succinate showed radioprotection and radiomitigation activity in mice. 5-AED and CBLB502 were also tested in nonhuman primates (NHP) and found effective. We are investigating the ability of these countermeasures to modulate blood cytokine levels with the objective of identifying cytokines as biomarkers of drug efficacy against radiation damage. Mice and NHP serum/plasma samples from irradiated and drug treated animals were evaluated using multiplex Luminex for quantification of cytokine concentrations. Our results in mice indicate that all radioprotective compounds stimulated G-CSF production. CBLB502 and CBLB612 induced maximum levels of G-CSF within 2-8 h, while the effect of tocopherol succinate was maximal 24 h post-injection. In addition to G-CSF, 5-AED stimulated production of IL-6; CBLB613, CBLB612 and CBLB502 stimulated production of both IL-6 and KC (keratinocyte derived chemokine). The induced cytokine spectrum of CBLB502 and CBLB612 was tested in NHP and found to be very similar to that in mice. In both species, G-CSF, IL-6, and KC levels were correlated with drug dose. The CBLB502 dose dependence of cytokine levels coincided with CBLB502 dose-dependent radioprotection in mice. In conclusion, our results suggest that specific cytokines may serve as biomarkers for efficacy of radiation countermeasures, and may prove useful as a predictor of outcome.