The distribution of molecular forms of serum IgA rheumatoid factor (IgA‐RF) in 42 patients with rheumatoid arthritis was examined by solid‐phase radioim‐munoassay following fractionation by gel chromatography or ultracentrifugation in acidic buffer. Analysis of the fractions using phosphate buffered saline indicated that the IgA‐RF in each serum was mainly polymeric. However, monomeric IgA‐RF was detected in sera from approximately two‐thirds of the patients, after dilution of chromatographic or ultracentrifugal fractions in diluent containing mouse monoclonal anti‐human α chain antibody. The levels of monomeric IgA‐RF (mean ± SD 38 ± 86 μg/ml) and the ratios of monomeric to polymeric IgA‐RF (mean ± SD 0.29 ± 0.41) varied over a wide range. Paired synovial fluids from 9 of the patients were also examined. Monomeric IgA‐RF was detected in each, although 2 samples demonstrated only minimal quantities. Neither form of IgA‐RF was detected in serum from healthy adults when analyzed under the same conditions. Thus, both monomeric and polymeric IgA‐RF can occur in serum and synovial fluid from patients with rheumatoid arthritis, and their proportions vary widely among patients. Copyright © 1986 American College of Rheumatology