Antipolynucleotide antibody from systemic lupus erythematosus patient serum was found to interact with common determinants of double-stranded DNA (dsDNA) complexed to protein within nucleosomes, and with protein-free denatured DNA. In contrast, purified dsDNA isolated from nucleosomes manifested decreased reactivity with antipolynucleotide antibody. These data suggest that immunoassays using denatured DNA antigen substrate are useful for identifying a broad spectrum of antipolynucleotide antibodies that are reactive with dsDNA complexed to protein.