A new method is described for quantitative determination of insulin antibodies. In this method, the insulin binding, capacity of non-iodinated insulin is determined. Diluted serum containing insulin antibodies was saturated by native insulin. Bound insulin in the incubated serum was fractionated by polyethyleneglycol(PEG). After the precipitated bound insulin was acidified, insulin antibodies were separated by PEG again and immunoreactive insulin in the supernatant was measured. The titer of insulin antibodies is expressed as microunits of insulin antibodies per milliliter of serum. The determination is not influenced by free or antibody bound serum insulin. The co-efficient of variation of insulin antibody titers' was 7.04% . There vwas a significant correlation (r=0.846, p<0.001) in serum antibody titers between our method and that of Welborn, but more sensitive results were obtained by our procedure. In patients with high bound insulin titers, reasonably accurate determinations were demonstrated. Acidification of samples was able to be performed at lower pH and longer reaction time was available. Another advantage of this method is that the changes in the titer of insulin antibodies in individual patients can be followed accuratly, because in other methods identical purity of hot insulin is not always available for each test. The diagnostic tool provided by this new method proved useful for diabetic patients being treated with insulin.