ABSTRACT A review of competitive protein binding methods for the determination of testosterone in plasma is given. The different steps discussed are: Choice of the binding protein, dilution of the protein solution, separation of the free from the bound fraction, purification of the extract and factors determining the blank. The method used in the author's laboratory is critically discussed. The recovery is 98% ± 4% at 1 ng. The coefficient of variation is 3% at 660 ng/100 ml, 13% at 35 ng/100 ml, and 40% at 20 ng/100 ml. The sensitivity is about 10 ng/100 ml.