The human insulin-like growth factor-1 (IGF-1) gene gives rise to multiple, heterogeneous mRNA transcripts by alternative splicing, thus producing different IGF-1 isoforms. The mechano growth factor (MGF) is an IGF-1 isoform that was found to be markedly up-regulated in exercised or damaged muscle. The specific E domain of the MGF splice variant may act as an independent growth factor. The aim of the present study was to characterize a rabbit antihuman MGF polyclonal antibody. New-Zealand rabbits were immunized by injections of a purified synthetic peptide corresponding to the last 24 amino acids of the human C-terminal of the MGF E domain. Western blotting and immunohistochemical techniques were used to characterize the specificity of the polyclonal anti-MGF antiserum. The anti-MGF antiserum was found to recognize the MGF E-peptide and not the common part of the IGF-1 isoforms, i.e. the mature IGF-1 peptide. Furthermore, it specifically bound to the MGF protein in human skeletal and in rat cardiac muscle, apparently due to the considerable homology between the human and rat MGF E-peptide sequences. Immunostaining analysis showed that this polyclonal anti-MGF antibody was able to detect MGF in human muscle and in rat cardiomyocytes and vessels' smooth muscle cells. We conclude that this rabbit polyclonal anti-human/rat MGF antibody could become a valuable tool in the study of IGF-1 isoforms in human and rat tissues.