IgA rheumatoid factor, IgA and IgG immune complexes were measured in 119 patients with IgA nephropathy. IgA rheumatoid factor was detected in 62/119 (52%) patients and in 92/265 (35%) serum samples. There was a good correlation (p less than 0.001) between the presence of IgA rheumatoid factor and the presence as well as levels of IgG immune complexes, but not between levels of IgA rheumatoid factor and other clinical or immunological parameters. However, higher levels of serum IgA were found in the subgroup of patients with constantly positive IgA rheumatoid factor. Using aggregated human IgG, we could not demonstrate antiglobulin activity in renal biopsy specimens from 36 patients. These results suggest that IgA rheumatoid factor does not play a primary role in renal damage in IgA nephropathy, but could simply reflect a response to IgG immune complexes in a disorder characterized by abnormalities of IgA production. Nevertheless, the presence of circulating IgA rheumatoid factor in a substantial proportion of patients, especially in those with features of polyclonal IgA activation, provides additional evidence for a general perturbation of IgA metabolism in this disease and could represent an antigen-specific system with which to study regulation of IgA synthesis.