Laser nephelometric estimation of IgM in the eluate fractions following Sepharose 6B chromatography has enabled the calculation of the proportion of low molecular weight IgM (7S IgM) in normal and pathological sera. This figure has then been used to determine the absolute amount of 7S IgM. Twenty-seven of 36 (75%) patients with rheumatoid arthritis had 7S IgM with a mean value of 17 mg/100 ml (170 mg/l) (range 2.5-59 mg/100 ml). No sera from 10 healthy controls were found to contain 7S IgM. Patients with active rheumatoid arthritis had significantly more 7S IgM than those with inactive disease, but there was no significant difference between those patients with and without rheumatoid vasculitis. Significant correlations occurred between 7S IgM and the absolute IgM level (P < 0.01), the Rose-Waaler titre (P < 0.01), and the erythrocyte sedimentation rate (P = 0.01). However, there was no significant correlation with the age of the patient, the duration of the disease, or the level of circulating immune complexes as measured by the Clq binding assay. It is concluded that 7S IgM commonly occurs in rheumatoid arthritis, and it is postulated that a common immunological stimulus is responsible for the production of 7S IgM and rheumatoid factors, serological abnormalities that characterise this disease.