The present study was performed to investigate the role of IgA rheumatoid factor (RF) in the formation of IgA-containing immune complexes and to determine the IgA subclass composition of IgA RF in patients with Henoch-Schönlein purpura (HSP). Immune complexes were isolated from the sera of 22 children with HSP and 13 controls by means of polyethylene glycol (PEG) precipitation. The percentage of IgG, IgA, and IgM precipitated by PEG was significantly greater in HSP patients than controls (p less than 0.01). There was a strong correlation (r = 0.723, p less than 0.001) between the amount of IgG and IgA in the PEG precipitates from HSP patients, but not controls. HSP patients had significantly higher levels of IgA RF in their serum (p less than 0.05) and in their PEG precipitates (p less than 0.05) compared to controls. There was a strong correlation between IgA RF concentrations in the serum and PEG precipitates in HSP patients (r = 0.910, p less than 0.001). PEG precipitation eliminated IgA RF activity from the serum of 7 of 8 HSP patients tested, and substantially reduced the titer in the remaining patient. IgA RF was recovered in the PEG precipitates from all patients. Testing of HSP patients showed that IgA1 was the predominant IgA subclass of the serum IgA RF (p less than 0.02) and PEG precipitate IgA RF (p less than 0.01). These results indicate that IgA RF is a constituent of IgA-containing immune complexes in HSP, and that IgA RF is composed primarily of IgA1.