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To investigate the temporal relationship of antibody responses to different La epitopes, sequential sera from nine patients with systemic lupus erythematosus and Sjogren's syndrome were tested by enzyme-linked immunosorbent assay for antibody binding to a series of recombinant fusion proteins containing different regions of the La molecule. The results of this analysis indicate that antibody responses to four different La fragments vary in parallel over time. This finding is supported by a statistical analysis indicating that the changes in antibody levels between the six pairs of responses were highly correlated (P less than 0.001). Furthermore, we show by immunoaffinity purification that antibodies to the three nonoverlapping La protein fragments do not cross-react with other fragments and, hence, represent independent populations. These results suggest that anti-La antibodies are coordinately produced to different epitopes on the La molecule, possibly reflecting an antigen-driven mechanism.