Monomeric IgM (7S IgM) is present in the serum of patients with certain inflammatory and neoplastic conditions. In the past it has been necessary to separate 7S IgM from 19S IgM by chromatography or ultra-centrifugation prior to its quantification, however, 7% agarose gels allow migration of 7S molecules while retarding the migration of larger (19S) molecules. We describe the development of a RID method on 7% agarose and detail the optimum conditions for the preparation of gels and standards in order to quantify 7S IgM present in serum in a single step without prior separation from 19S IgM. The major difficulties encountered were in the preparation of the agarose gels and the conditions necessary to overcome the difficulties are described. A linear relationship was found between the ring diameter squared and the 7S IgM concentration. The assay could detect as little as 15 micrograms/ml of 7S IgM. Within assay variability was 15.4%. In the sera of 45 normal individuals monomeric IgM was not detected whereas 15 (43%) of an unselected group of patients with rheumatoid arthritis had measurable levels of 7S IgM. This method for quantifying 7S IgM is both simple and inexpensive, and because of the ease with which large numbers of sera can be processed in a single assay it is less time consuming than other methods used to quantify 7S IgM.