SummaryAn improved procedure has been described for measurement of interaction of globulin from lupus erythematosis serum with nucleoprotein, or artificial complexes of desoxyribonucleic acid and histone. Relatively large glass surface areas coated with these materials bound maximum amounts of the serum factor, and allowed most efficient removal of unbound serum protein by washing. Antibody for human gamma globulin labelled with iodine131 provided a sensitive means of measuring amount of bound globulin when used under proper conditions. Measurement of relative concentrations of serum factor was achieved in test system employing a single tube of serum at 20% concentration.