A simple routine method is described for the quantitation of antiglobulin (rheumatoid factor) in human serum irrespective of its immunoglobulin class. Papain Fc fragment of human IgG is labelled with fluorescein isothiocyanate and incubated with the serum. Bound Fc is separated from free Fc by precipitation with polyethylene glycol and measured by the fluorescence of the precipitate. Results are expressed as a percentage of the binding obtained with a normal pool serum. About 60% of seronegative rheumatoid sera and 96% of seropositive rheumatoid sera gave results more than 2SD above the mean value for normal sera. Only 17% of osteoarthritis sera gave positive results by the same criterion. A relationship is also deduced to show that if the test is carried out at a number of Fc concentrations a double reciprocal plot of bound Fc against free Fc enables both the total antiglobulin and the ratio of binding constants of antiglobulin for Fc and for IgG to be calculated. This ratio is approximately unity. Under the standard conditions the results correspond to a measurement of approximately 30% of the total antiglobulins present.