Laser nephelometry is a sensitive technique for measuring the degree of light scatter caused by the interaction of rheumatoid factors with heat aggregated human IgG. In measuring the rheumatoid factor (RF) level in rheumatoid serum and synovial fluid, it was found that nephelometry measured the combined amounts of IgM-RF and IgG-RF. However, on an equivalent weight basis, IgM-RF produced at least 17 times more scatter than IgG-RF. Low molecular weight IgM-RF had minimal light scattering activity in this assay. The total serum RF level, measured by nephelometry and expressed in terms of microgram/ml of IgM-RF equivalents, correlated strongly with IgM-RF measured by solid phase radio-immunoassay (r = 0.97, n = 30, P less than 0.001) and an equation was derived to equate the two variables. It is concluded that the nephelometric RF test is a simple and rapid method to measure the total polyclonal RF level in various biological fluids, although monoclonal or an occasional polyclonal preparation of rheumatoid factor may give atypical results.