Polyclonal antibodies to double stranded DNA (dsDNA) purified from pooled serum samples of patients with systemic lupus erythematosus (SLE) exerted cytotoxic effects on cultured rat mesangial cells. At concentrations from 5 to 150 IU/ml, antibodies to dsDNA inhibited the incorporation of thymidine labelled with 3H into rat mesangial cells in a dose response manner after three days of culture. In contrast, normal human IgG (1 mg/ml), heat aggregated human IgG (1 mg/ml), N-formyl-methionyl-leucyl-phenylalanine (1 x 10(-7) mol/l), tumour necrosis factor alpha (16 U/ml), lipopolysaccharides (1 microgram/ml), 4 beta-phorbol-12 beta-myristate-13 alpha-acetate (PMA) (20 ng/ml), interleukin 1 beta (10 U/ml), and 20% v/v phytohaemagglutinin stimulated mononuclear cell supernatant showed no significant effect on these cells. Anticardiolipin antibody, another autoantibody purified from the serum of patients with SLE, also inhibited the proliferation of rat mesangial cells but to a lesser extent. In the presence of antibodies to dsDNA (100 IU/ml), the mesangial cells became spherical and clustered together, which was very different from the original stellate appearance. These autoantibodies also depolarised the membrane potential of mesangial cells. Antibodies to dsDNA decreased the syntheses of prostaglandin E2, 6-keto-prostaglandin F1 alpha and thromboxane B2 by mesangial cells. In an in vivo study, the antibodies to dsDNA showed a strong affinity for the glomeruli when intravenously injected into rats. These results suggest that the nephrotropic antibodies to dsDNA can directly damage the glomerular mesangial cells in addition to the formation of immune complexes with DNA which may cause kidney inflammation and tissue destruction.