The levels of 'putative' anti-idiotypic antibodies reactive with F(ab')2 fragments of affinity purified DNA binding antibodies from five SLE patients were measured in the serum of active and inactive systemic lupus erythematosus (SLE) patients, their relatives, spouses and healthy non-related individuals. Serum anti-idiotypic antibodies of the IgG isotype could be detected in the serum of 8/15 inactive SLE patients, 3/19 active SLE patients, 17/27 SLE relatives, 1/4 SLE spouses and 6/32 healthy non-related individuals. When the serum immunoglobulins from the individuals in each group were separated into IgG and IgM antibody fractions and analysed, a significant increase in levels of IgG anti-idiotypic reactivity were found in healthy individuals (16/32), whilst no significant increase in IgG anti-idiotypic antibodies were detected in SLE patients, SLE relatives and SLE spouses. Comparatively few individuals tested showed detectable levels of IgM anti-idiotypic antibodies. These results demonstrate that anti-idiotypic antibodies reactive with anti-DNA antibodies are detectable in the serum of SLE patients, SLE relatives, SLE spouses and healthy individuals, and are predominantly of the IgG isotype. The increased frequency of IgG anti-idiotypic antibodies after separation from serum IgM antibodies infers that anti-idiotypic activity of some IgG immunoglobulins in the sera of healthy individuals may be masked by the presence of IgM antibodies in the serum.