The lactulose and HMF contents in market milks were analyzed by gas chromatographic and colorimetric methods, respectively. UHT milk samples, directly or indirectly heat-treated, were also examined for lactulose and HMF during a period of 120 d storage in 4-week intervals at room temperature or 30 degrees C. The gas chromatographic method was Evaluated for precision and accuracy using phenyl-beta-glucoside as internal standard. The mean recovery was found to be 99.1%. The mean lactulose content of the market milks determined was 1.2-1.9 mg/100 mi in pasteurized milks and 7.6-45.4 mg/100 ml in UHT milks and were found to be within the limits considered acceptable. The HMF contents of UHT milks varied between 5.2 and 15.1 mu mol/l. The lactulose content has been shown to be useful in distinguishing pasteurized, directly heated UHT and indirectly heated UHT milks. The production dates of UHT milks were found to be important in definition of the lactulose level because of the increase of lactulose during the storage at 30 degrees C.