SummarySince the normal circulating level of thy‐roxine is so much lower in canines and other species than it is in humans, the authors suggest that it is impractical to use systems designed for thyroxine detection in human sera on their veterinary patients. Their newly developed, simple yet sensitive radioimmunoassay is presented here and further outlined in the accompanying table. Successfully performed on 500 canine patients, this radio‐immunoassay features the use of T4standards prepared with T4‐free serum, I125thyroxine with a very high specific activity, and specific antiserum for thyroxine, previously titered to give 50% binding of radioactive T4in the test system. Polyethylene glycol is used to separate the bound and free radioactive T4, resulting in a procedure which can be performed rapidly at room temper