An early endosome antigen previously reported by F.T. Mu could be stained on cytoplasmic vesicles of HEp-2 cells. Here, we have investigated autoantibodies against cytoplasmic vesicular antigens, especially against early endosome antigen 1. Twelve sera were selected on the basis of cytoplasmic vesicular staining patterns of HEp-2 cells. Protein-immunoprecipitation using 35S-methionine labeled HeLa lysates, and RNA immunoprecipitation using 32P-labeled HeLa lysates were conducted to characterize the cognate antigens. Nine of 12 sera reacted with proteins in the range of 162-180 kDa, three of which were found to react specifically with the 162 kDa 35S methionine labeled recombinant early endosome antigen 1. These proteins were not associated with common RNA. Although complete clinical information was not available, some of the patients had rheumatoid arthritis(RA). In addition, the RNA-IPP results suggest that other patients included one each with SLE, SSc, polymyositis, and Sjögren's syndrome. Anti-early endosome antigen 1 antibody was found in 25%(3 of 12) of sera known to stain cytoplasmic vesicles. The reactive sera came mostly from patients with RA. The sera was from one case each of clinical-confirmed RA, SLE and Sjögren's syndrome.