Using indirect immunofluorescence methods on rat liver sections and HEp-2 cells, combined with immunoprecipitation in agar, we were able to detect antinuclear antibodies in 76% of sera from 42 patients with polymyositis or dermatomyositis. In addition, 7% of the patients had anti-cytoplasmic antibodies without antinuclear antibodies. With immunofluorescence, the most common pattern was a nuclear fluorescence that was homogeneous on rat liver sections (11 cases) and speckled on HEp-2 smears (16 cases). The frequency of antinuclear antibodies was the same in the different categories of myositis as classified by Pearson. Precipitating antibodies were observed in 15 cases (36%). Two specific antibodies were frequently encountered: anti-Jo1 in 8 cases (19%) and anti-Pm-Scl in 5 cases (12%). Anti-Jo1 antibodies were present in both polymyositis and dermatomyositis. Seven out of the 8 patients with anti-Jo1 antibodies had pulmonary fibrosis (P less than 0.03).