ABSTRACT: The purpose of the study was to investigate the effect of IFN-γ on transcriptomic profile of differentiating mouse C2C12 myogenic cells. Global gene expression was evaluated using the oligonucleotide whole mouse genome microarrays and was validated with real-time PCR method. Exogenous IFN-γ (1 ng/ml) increased myoblast proliferation, but decreased cell viability, the fusion index and the cellular content of myosin heavy chain, MyHC in C2C12 cultures on the 3rd day of differentiation. IFN-γ up-regulated genes were mainly involved in biological processes such as: cell cycle, regulation of cell proliferation, programmed cell death, inflammatory, vasculature development, regulation of cytokine, transmembrane receptor protein tyrosine kinase signaling pathway, and chemotaxis, whereas down-regulated genes contributed mainly to: regulation of transcription, cell-cell signaling, nitrogen compound biosynthetic process, transmembrane receptor protein ser/thr protein kinase signaling pathway, and regulation of Wnt receptor signaling pathway. IFN-γ up-regulated the expression of cytokines/growth factors controlling cell proliferation (Cxcl10, Il15, Ccl2, Fgf7, Figf, Csf1, Vegfc, Hgf). Moreover, IFN-γ: i) down-regulated genes encoding factors that are anabolic for muscle cells (Fst, Igf-1); ii) inhibited pro-myogenic transcription (via Mef2a, Nfkb1, and Pparg); iii) decreased expression of genes controlling cell adhesion and sarcolemma/cytoskeleton organization; and iv) activated the proteolytic pathways: proteasomes and catepsins, leading to protein degradation and impaired myotube growth. Our data suggest that the effect of IFN-γ on mygenesis is, at least partly, associated with the regulation of muscle cell secretom at the transcriptional level. To whom the correspondence should be addressed: Dr K. Grzelkowska-Kowalczyk; e-mail: k_grzel_kow@poczta.fm, tel/fax: (48 22) 847 24 52