SUMMARY
A method is described for the measurement of testosterone in peripheral blood plasma by competitive protein binding, using a 1:80 dilution of human late-pregnancy plasma and Sephadex for separating protein-bound from unbound steroid. The reliability criteria were examined in detail for plasma from bulls and rams. Testosterone was measured with a high degree of accuracy, and with a precision of 4·9–7·9% in the concentration range of 4–24 ng/ml. The specificity of the method was checked by comparing it with a gas-chromatographic technique, and the agreement was good. The testosterone concentrations in the peripheral plasma of bulls and rams were found to range between 2 and 24 ng/ml.