Soy isoflavones were quantified from human milk by a fast, precise, and selective HPLC method after enzymatic hydrolysis of conjugated isoflavones and extraction with ethyl acetate. Isoflavone aglycones and their mammalian metabolites equol and O-desmethylangolensin were separated selectively and identified by absorbance patterns, fluorometric and electrochemical detection, gas chromatography-mass spectrometric analysis after trimethylsilylation, and with internal and external authentic standards. HPLC injections of 20 microL of human milk showed detection limits of 1-3 pmol for all analytes by using diode-array detection. The detection limit could be improved by as much as 1000-fold by extended concentration through partitioning with ethyl acetate, by using electrochemical detection, by increasing the injection volumes, or by combining these techniques. We used the proposed method to monitor isoflavone concentrations in human milk and in human urine after challenge with 5, 10, and 20 g of roasted soybeans in the diet. Implications of the results for the potential of isoflavones to prevent cancer in newborn infants exposed to these agents are discussed.