The U1 snRNP (small nuclear ribonucleoprotein complex) associated 70K protein is the main autoantigen for the anti-RNP autoantibodies which are directed against the U1 snRNP particle. The major antigenic region of the 70K protein has by various laboratories been mapped to an RNA binding domain required for the 70K-U1 snRNA interaction. We have used recombinant proteins comprising this region from the human and the Drosophila melanogaster 70K proteins to examine the species specificity of the human anti-70K autoantibodies found in 42 patient sera. Most, but not all, anti-70K positive sera in this cross-sectional sample contained both human 70K specific anti-bodies and Drosophila 70K reactive antibodies. Results of a longitudinal follow-up of 14 patients indicated that the cross-reactive anti-70K antibodies developed secondarily to the establishment of a species-specific anti-70K reaction. In a fraction of the patient sera this broadening of the response never occurred. Taken together, the data in this study support the hypothesis that the endogenous human 70K protein is the immunogen driving the production of anti-70K autoantibodies.