A radioimmunological method for the detection and measurement of insulin antibodies in human sera was developed using polyethyleneglycol (PEG) as separation agent. Studies were performed on 150 sera of nondiabetic subjects and on 27 sera collected from diabetic patients treated with insulin. The per cent binding of 125I-insulin in normal sera ranged similarly to control free-insulin serum while in sera from insulin-treated patients8 the binding was significantly greater. The technique is quick (24 hours) and the intra-assay precision (correlation coefficient for replicates) varies from 0.87 to 0.99%. The separation technique using PEG for the precipitation of the antigen-antibody complexes may be extended to the measurement of antibodies for any other substance of lo, molecular weight (under 10,000 daltons).