We have isolated and characterized a novel antibody from a patient with polymyositis, which precipitates threonine tRNA and an unknown small RNA molecule from 32P-labeled HeLa extract. Fingerprint analysis showed that the human threonine tRNA precipitated was nearly identical to the murine tRNA described by Harada [(1978) Seikagaku 50, 397-411]. Nucleotide analysis suggested that the other small RNA molecule might be transfer RNA. Since phenol extraction resulted in the loss of immunoprecipitability, the protein portion was presumed to be involved in the recognition of the antigen. Immunofluorescence staining of HeLa cells with the antibody clearly demonstrated that the antigen was located predominantly in the cytoplasm.