Abstract Rationale One of the most feared space flight conditions is radiation due to the potential to weaken the immune system. Leukocyte studies in space demonstrated a reduction in proliferative responses, alterations in signal transduction and cytokine production, along with reactivation of latent viruses. Utilizing our previously developed human B cell in vitro model, we present evidence of space-like gamma radiation-induced B cell defects. Methods CD40-activated purified human peripheral blood B cells were exposed to varying levels of gamma radiation (1-6 Gy) delivered by a 137Cs source. Determination of B cell responses and apoptotic processes were made by flow cytometric analysis, ELISA, colorometric, protein, and proliferative assays. Results Human B cells were significantly altered by 3 Gy gamma radiation (estimated radiation absorbed by humans on a two-year space journey). There was significant reduction in cell viability (40%), proliferation (70%), and CD79b (B cell receptor) expression (50%) ( p<0.01). B cell apoptosis was induced as evidenced by increased Annexin V binding (95%), pro-apoptotic receptor expression (Fas/CD95) (70%) and the anti-apoptotic oncogene (Bcl-2) (120%) ( p<0.01). At 6 Gy radiation, only 50% viable cells remained. As a first attempt countermeasure, the general caspase inhibitor, z-VAD-fmk, reversed CD95 expression while exerting no significant effect upon Bcl-2. Conclusion Exposure to gamma radiation significantly altered normal human B cell responses to defined ligands and induced enhanced apoptosis in B cells. Thus, these data will aid in the evaluation of the extremes of radiation that humans are likely to be exposed to in space exploration.