1. Comparative studies on the stereochemistry of the metabolism of isoprene in vitro have been carried out using liver microsomes from rats, mice, monkeys, dogs, rabbits and humans. Differences between strains and gender were also investigated. 2. In the production of the isoprene monoepoxides,microsomes from the livers of the male Sprague- Dawley or Wistar rat showed an approximately 2:1 preference for the formation of (S)-2-(1-methylethenyl)o xirane compared with the (R)-enantiomer. No enantioselectiv ity was observed for mouse or rabbit. In contrast, liver microsomes from dog, monkey or male human preferentially formed (R)-2-1(1-methylethenyl)axirane. There was no enantioselectivity observed with microsomes from female human liver. 3. The significant differences between species in the in vitro metabolism of isoprene indicate that stereochemical and mechanistic data should be taken into account when evaluating the results of animal studies designed to assess the carcinogenic risks to humans that may be associated with exposure to isoprene.