Using radial immunodiffusion in 7% agarose, 7S IgM was quantified in the sera of 45 normal individuals, 37 patients with rheumatoid arthritis (RA), 18 patients with psoriatic arthritis and 11 patients with ankylosing spondylitis. 7S IgM was only found in the sera of patients with RA, 43% of whom had detectable levels of 7S IgM (median 47.5 micrograms/ml). The patients with 7S IgM had significantly higher IgM rheumatoid factor (IgM RF) and C-reactive protein levels in their sera (p less than 0.005). There was a strong correlation between 7S IgM and IgM RF levels in the sera of these patients. These data demonstrate that patients with more active and severe disease have 7S IgM present in their sera but the absence of 7S IgM from the sera of some patients with high levels of IgM RF and CRP suggest that additional factors may influence the synthesis and secretion of 7S IgM by B cells in RA.