Antinuclear antibodies (ANA) of the IgE class were studied in sera from patients with rheumatoid arthritis (RA), systemic lupus erythematosus (SLE) and healthy controls. Sixty per cent of 20 RA patients with neutropenia were found to have IgE granulocyte-specific (GS-)ANA, whereas only 16% of RA patients without neutropenia had IgE antibodies of similar specificity. About 5% in each group of RA patients had IgE organ-nonspecific (ON-)ANA. Eleven of 15 patients with active SLE and only 4 of 20 with inactive SLE had IgE ON-ANA. Sera from five patients with lupus nephritis all contained IgE ON-ANA. None of 100 sera from controls showed presence of IgE ANA. IgE ANA titres in RA and SLE patients correlated to the titres of ANA of the other four immunoglobulin classes. Gel filtration studies at neutral and acid pH of RA sera containing high titres of IgE GS-ANA indicated the presence of these antibodies in immune complexes. Studies of serum cryoprecipitates supported this conclusion. IgE ANA production may be of pathogenetic importance in RA and SLE by eliciting type-I reactions.