Antibodies specific for ribosomal P proteins (anti-P) are a hallmark of systemic lupus as anti-DNA antibodies are. It has been reported that anti-P antibodies are more frequently detected in anti-dsDNA positive sera. The aim of the present study was to verify the binding ability of anti-P antibodies towards polynucleotides and nucleosomes. We purified anti-P antibodies from 8 SLE patients' sera and we analysed them by ELISA on plates coated with DNA or nucleosomes. We performed also inhibition experiments in order to verify the specificity of the binding. All the purified anti-P antibodies bound DNA, but some anti-DNA binding activity remained among the non-anti-P antibodies in the flow through. Only half of the purified antibodies bound to nucleosomes, and anti-nucleosomal activity was demonstrated also in non anti-P antibody fraction. The inhibition experiments performed on two anti-P antibodies pointed out that only the homologous antigen inhibited the binding to either P peptide or DNA coated onto the solid phase. These results show that sera in which the two specificities coexist contain antibodies endowed with a double binding ability for DNA and the P peptide.