Antibody binding to total protein extracted from a mammalian source (HeLa cells) and from a prokaryotic source (Escherichia coli) was compared in sera from patients with systemic lupus erythematosus (SLE) and sera from normal subjects. When the average numbers of peptides or proteins recognized by IgG antibodies were compared on immunoblots, SLE sera bound to a significantly greater number of proteins from the HeLa cell extract than did sera from normal individuals (P less than 0.001). In contrast, SLE sera actually bound to fewer E coli proteins than did the sera obtained from normal controls, although the difference was not statistically significant. There was no correlation between the number of E coli proteins and HeLa proteins recognized by individual SLE sera, and there was no trend toward reactivity with a larger number of antigens in sera containing higher levels of IgG. IgG from SLE sera did not bind to 6 purified eukaryotic protein standards (selected solely on the basis of differences in size and charge) either in their denatured state or in their native state. These findings indicate that the high levels of IgG antibodies against selected eukaryotic intracellular proteins in patients with SLE cannot be explained by a random polyclonal B cell activation.