The human derma emits volatile compounds whose interaction with a receiver's olfactory sensory system may affect individual recognition and mating preferences. Studies suggest that both genes and environmental factors determine characteristic odor of an individual. We used solid-phase microextraction and gas chromatography-mass spectrometry to identify 3-methylbutanal in human axillary odor; we showed that the abundance of this volatile compound varies significantly among individuals and demonstrated that its formation in vitro may be influenced by interaction between human leukocyte antigen peptide and dermal microflora.