A method is described in detail for the assay of insulin in plasma or serum. It involves titration with labeled insulin of reactive insulin antibodies remaining after preincubation of the sample under assay or of standard solutions of insulin with an excess of guinea pig anti-insulin serum. Labeled insulin bound by antibodies in the assay is precipitated in the presence of alcohol (75 per cent, v/v) at room temperature, unbound labeled insulin then remaining in solution. The technic is simple and assays can be carried out in a few hours. It is shown that individual results are amenable to statistical assessment and suggested that the sensitivity of the method is only limited by the specific activity of the labeled insulin used.

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