A method is presented for the colorimetric assay of para-aminobenzamidine (PAB) in blood. Essentially, the PAB in trichloroacetic acid extracts is coupled to N-(l-napthyl)ethylenediamine in a typical diazotization reaction. The colored complex formed exhibits a peak absorption at 540 nm. This complex in distilled water is very stable and loses less than 5% of its absorptivity over a 24 hour period. Trichloroacetic acid extracts of whole blood containing PAB are likewise stable, exhibiting less than a 7% mean reduction in absorbance after standing for 24 hours. A plot of optical density versus concentration of the PAB diazo complex over a range of 1–20 μg/ml shows excellent linerarity. The method can accurately quantitate as little as 1 μg/ml at a dilution of 3.5.