Two hybridoma clones secreting monoclonal antibodies (MAbs) to mechano-growth factor (MGF) have been produced by cell fusion technique. Isotyping of the MAbs revealed that both belong to the G1 subclass. The epitope specificity of the MAbs has been examined in competition experiments. No competition was detected, suggesting that the MAbs obtained recognize different antigenic determinants. MAbs of one clone (8B9) recognize human MGF peptide absent in insulin-like growth factor-1 (IGF-1) and comprising amino acids from 87 to 111. Affinity binding constants with the full-length MGF and 87-111 amino acid peptide have been determined by enzyme-linked immunosorbent assay (ELISA). A pair of monoclonal antibodies obtained can be used in a sandwich-type assay to quantify MGF.