Putative cross-reactions between anti-DNA and anticardiolipin activities were studied using sera of different patients and a panel of monoclonal antibodies to DNA. Sera were obtained from patients with systemic lupus erythematosus, from patients with syphilis, and from heroin addicts who showed a biologic false-positive result on the serologic test for syphilis. While the patients with syphilis and the heroin addicts had elevated levels of anticardiolipin antibodies in their circulation, no reactivity with DNA was observed in these sera. Sera from systemic lupus erythematosus patients often showed both anti-DNA and anticardiolipin activity. Although a correlation between anti-DNA and anticardiolipin titers was found, this did not always result from cross-reactivity of the same population of antibodies. In fact, we observed a relationship between cross-reactivity and antibody avidity. Anti-DNA of high avidity to DNA showed little cross-reactivity with cardiolipin. Anticardiolipin activity in such sera was based on the presence of specific anticardiolipin antibodies. Anti-DNA of low avidity was found to cross-react with cardiolipin. Among monoclonal antibodies to DNA, we found that cross-reactions with cardiolipin were rare: only 6 of 55 anti-DNA clones produced antibodies that also reacted with cardiolipin.