Patients with systemic lupus erythematosus and other rheumatic diseases often possess autoantibodies directed against discrete classes of small ribonucleoprotein particles (RNPs). The class of particles recognized by anti-Ro antibodies contains from two to four small cytoplasmic RNAs, depending on the mammalian species examined. We find that an antigenic polypeptide of 60 kDa is the major protein residing in Ro RNPs from human HeLa cells. To determine what common feature of Ro RNA sequence or structure is recognized by the Ro protein, we carried out ribonuclease protection experiments on isolated Ro RNPs from HeLa cells. For each of the three human Ro RNAs whose sequence is known, the most highly protected portion found in immunoprecipitates corresponded to the lower section of a stem formed by base-pairing the 5' and 3' ends of the RNA. Within this protected helix is a highly conserved region composed of seven identical base pairs with a single bulged cytidine. We discuss possible functions for the Ro RNPs.