The intestinal microbial transformation of daidzein into equol is subject to a wide inter-individual variability. The aim of this study was to investigate in vitro this transformation and to evaluate possible correlations between individual diet and equol production. The transformation of daidzein was investigated in anaerobic batch cultures inoculated with mixed fecal bacteria from 90 volunteers. The daidzein metabolism was monitored by liquid chromatography-mass spectrometry, and a chiral column was used to distinguish equol and dihydrodaidzein enantiomers. The obtained results show that daidzein was unchanged ( approximately 27%) or degraded to equol ( approximately 28%), O-desmethylangolensin ( approximately 12%) or dihydrodaidzein ( approximately 31%). Furthermore, some subjects ( approximately 2%) are able to produce both equol and O-desmethylangolensin. Bacteria represent sub-dominant populations (10(5)-10(9) cell/g wet faeces) in "slow" equol producers, while higher counts of equol-producing microorganisms (10(10)-10(11) cell/g wet faeces) were found in "quick" equol producers. The in vitro test to evaluate equol-producing status is quick and not invasive, and the obtained results are comparable with those reported in vivo. Indeed, the only enantiomer present in the batch cultures containing equol was the S-form. No significant correlations between equol production, BMI, age and sex were found. It seems that the equol-producer group consumed less fibre, vegetables and cereals, and more lipids from animal sources.