Determinations of anti-histone antibodies (AHA) by ELISA were carried out in 109 cases of SLE, 117 of RA, the positive rate being 50.5%, 23.1% respectively, with titres in SLE patients higher than in RA. AHA was 90.2% positive in active cases of SLE patients. SLE patients with AHA showed a higher incidence of pericarditis and arthritis, but a lower rate of malar rash than SLE patients without AHA. In RA, there is a higher incidence of extraarticular manifestations in AHA positive patients IgM-AHA was this predominant AHA in RA while in SLE patients it was the IgG-AHA. For SLE, IgG-AHA which was more closely associated with anti-ds DNA was more significant than IgM-AHA.