We used immunocytochemistry to identify ubiquitin and beta-amyloid-protein in muscle biopsies from patients with three neuromuscular disorders characterized by the presence of rimmed vacuoles in muscle fibres: inclusion body myositis (IBM), familial IBM-like disorder and oculopharyngeal muscular dystrophy (OPMD). Labelling with anti-ubiquitin antibodies was observed in all three diseases, but it was frequent in IBM, less common in familial IBM-like disorder and rare in OPMD. This labelling is thought to correspond to the presence of IBM-type filaments (16-18 nm in external diameter) which are characteristic but not specific for IBM or familial IBM-like disorder, as they may also occur in other diseases including OPMD. Labelling with anti-beta-amyloid-protein antibody was seen in a few fibres in IBM but not in the other two conditions. The structures labelled with this antibody have yet to be determined. Labelling with anti-ubiquitin or anti-beta-amyloid-protein antibodies was not correlated with the presence of acid phosphatase activity.