Radiation-induced bystander effects have been studied extensively due to their potential implications for cancer therapy and radiation protection; however, a complete understanding of the molecular mechanisms remains to be elucidated. In this study, we monitored transcriptional responses to γ radiation in irradiated and bystander fibroblasts simultaneously employing a genome-wide microarray approach to determine factors that may be modulated in the generation or propagation of the bystander effect. For the microarray data we employed analysis at both the single-gene and gene-set level to place the findings in a biological context. Unirradiated bystander fibroblasts that were recipients of growth medium harvested from irradiated cultures 2 h after exposure to 2 Gy displayed transient enrichment in gene sets belonging to ribosome, oxidative phosphorylation and neurodegenerative disease pathways associated with mitochondrial dysfunctions. The response to direct irradiation was characterized by induction of signaling and apoptosis genes and the gradual formation of a cellular immune response. A set of 14 genes, many of which were regulated by p53, were found to be induced early after irradiation (prior to medium transfer) and may be important in the generation or propagation of the bystander effect.