Our objective was to identify nuclear calreticulin-binding protein(s) and investigate whether there is a correlation between presence of autoantibodies against calreticulin and calreticulin-binding protein(s) in the sera of patients suffering from systemic lupus erythematosus (SLE). The ligand overlay procedure using digoxigenin-labelled calreticulin was used to identify a calreticulin-binding protein in the nuclear fraction of bovine brain. Fractionation of the nuclear components was used to localize the major positive calreticulin-binding protein. The protein was partially purified using hydroxylapatitie chromatography and subjected to NH2-amino acid sequence analysis. Immunoblots using the sera of SLE patients were then carried out on calreticulin and the calreticulin-binding protein. The calreticulin-binding protein present in the nucleoplasm was identified as histone H1. Approximately 62% (26/42) patients with SLE had IgG antibodies directed against H1 whereas the sera of healthy individuals did not react with the antigen; 36% of patients with SLE had both anti-calreticulin and anti-histone H1 antibodies. Phosphorylation of the latter protein did not alter its immunoreactivity. These findings demonstrate that the concomitant presence of autoantibodies directed against both calreticulin and histone H1 occurs frequently in patients with SLE and may help shed some light on the mechanisms which bring about the autoimmune response.