Serologic studies on 114 patients with polymyositis and dermatomyositis revealed that 89% had either a precipitating antibody to antigens in calf thymus extract or a positive immunofluorescent reaction on HEp-2 cells, a human tissue culture line. Previously, the greatest proportion of polymyositis sera demonstrating positive serologic results (i.e., the proportion of patients' sera forming precipitates with calf thymus extract) was reported to be 60%. Use of the HEp-2 cell as immunofluorescent substrate enabled the detection of antibody in 89 (78%) of the sera, providing the additional probe which demonstrated specific antibody. Nuclear, cytoplasmic, and nucleolar staining are the most common patterns of immunofluorescence. The immunofluorescent patterns and individual precipitin reactions are related to each other and to the clinical syndromes in which they appear.