The occurrence and persistence of diacetyl in samples of unfermented raw and processed milk have been investigated; in addition, samples of milk fermented by different lactic acid bacteria have also been analysed. Diacetyl was determined by using a simplified gas chromatographic method in which acetone was the extracting agent. Diacetyl in unfermented raw milk of cow, buffalo, goat and sheep was found to be 45.1, 65.2, 42.8 and 41.2 mg kg(-1), respectively. The pre-fermentative diacetyl in milk was highly stable, and its initial value irrelevantly changed after the milk had been stored at 20 A degrees C for 24 h or freeze-dried for 8 h and finally, having been thermically treated at 80 A degrees C for 30 min. Significant amounts of diacetyl were detected also in all the processed milks. At 26 A degrees C growth temperature, the tested lactic acid bacteria produced diacetyl in milk in the following order: Streptococcus thermophilus < Lactobacillus paracasei < Lactobacillus rhamnosus. Diacetyl increased during fermentation of 24 h, while a spontaneous and significant loss was registered when fermentation was prolonged to 48 h.