To determine the specificity of autoantibodies for various antigenic sites on a self-protein molecule, sera from 19 patients with anti-La antibodies were tested for their reactivity with molecularly cloned La protein fragments. By quantitative ELISA, anti-La sera from patients with various connective tissue diseases were shown to react with La fusion proteins containing different regions of the La molecule. Two recombinant La fragments containing the carboxyl three-fourths and the middle one-third of the La sequence, respectively, bound higher levels of anti-La antibodies than the two fragments representing the amino and carboxyl terminals. Purified bovine La protein effectively competed for the binding of human autoantibodies to three of the four recombinant La fusion proteins, suggesting similarity in antigenic presentation between the La epitopes in these fusion proteins and the native La molecule. Immunoadsorption experiments showed that most anti-bovine La protein antibodies were removed from a human serum by affinity chromatography by using the fusion protein containing the carboxyl three-fourths of the La sequence, thus supporting the results obtained by quantitative solid phase ELISA. These studies demonstrate that anti-La autoantibodies recognize three La fragments representing separate nonoverlapping regions of the La sequence and are compatible with a mechanism of autoantibody production based on an immune response to the entire self-protein molecule.