Masticatory muscle myositis (MMM) is presumed to be an immunologically mediated canine myopathy but is of unknown origin. Severe atrophy and degeneration of masticatory muscle fibres, infiltration of eosinophilic granulocytes, and proliferation of the fibrous interstitial tissue are the hallmarks of MMM. Transforming growth factor-β (TGF-β) is a multifunctional regulatory peptide controlling myogenesis, inflammation and tissue repair. We investigated immunocytochemically the expression of TGF-β1 and latent transforming growth factor-β binding protein (LTBP), a TGF-β modulator protein, in cases of MMM. The study demonstrated the presence of TGF-β and LTBP in muscle fibres, infiltrating leucocytes and extracellular matrix in MMM, and suggested that TGF-β and LTBP play a role in muscle tissue repair, inflammation and fibrogenesis in MMM. © 1995 Academic Press Limited.