Glia maturation factor-beta (GMFB) is a 17-kDa protein that was initially identified as a growth and differentiation factor acting on neurons as well as glia in the vertebrate brain. We isolated human and rat glia maturation factor-gamma (GMFG) cDNA and examined the tissue distribution of GMFG in human and rat by Northern blots and Western blots. Sequence analysis of the entire cDNA revealed an open reading frame of 426 nucleotides with a deduced protein of 142 amino acid residues. The deduced amino acid sequence of its putative product is highly homologous to GMFB. Northern blot analysis indicated that a 0.9 kb mRNA is predominantly expressed in rat thymus, testis, and spleen. In comparison with GMFB, the current study demonstrated that the tissue distribution of GMFG is not the same as that of GMFB, and GMFG is predominantly in proliferative and differentiative organs.