A solid-phase assay for anti-histone antibodies is described, in which antibodies bound to calf thymus total histones, specific histone classes, or chromatin were detected by radioactive anti-human IgG, IgA or IgM. The reactivity of human sera from various rheumatic diseases was analysed by this assay and compared with results obtained from an indirect immunofluorescence assay (FANA) previously described. Sera that were positive in the FANA assay were usually positive in the solid-phase assay when total histones were the solid-phase antigen. However, many sera had IgM antibodies to total histones in the solid-phase assay but gave no detectable reaction in the FANA assay. Results from the solid-phase assay, using individual histones and chromatin, showed that the anti-histone antibodies in these sera were predominantly reactive with histones H3-H4 and did not bind well to histones complexed to DNA in the form of chromatin.