This work establishes the optimum conditions for the protein nitrogen unit (PNU) phosphotungstic acid (PTA) precipitation procedure for allergenic extracts. The volume of extract analyzed, the dilution of the extract prior to precipitation, the percent PTA in the precipitating solution, the washing of the precipitate, and the volume of concentrated HCl initially added to the extract were optimized, as they were found to be the factors which affected the amount of PTA precipitate obtained. Varying the digestion time, the digestion temperature, and the percent HCl in the 15% PTA precipitating solution did not cause detectable differences in the PNU values of the extracts studied.