The use of microorganisms as tools in the study of drug metabolism appears to be gaining popularity. The selected examples cited here provide additional evidence of the utility of these systems as alternative in vitro models for studying drug metabolism in humans. However, as was noted earlier, this model, nor any other in vitro model system could ever replace animals in biomedical research. However, it is apparent from the numerous examples cited here and in the previous review of this area that microorganisms are a reliable, reproducible alternative to small animals as predictive models in drug metabolism studies. The continuing development of techniques that reduce the use of animals in research is encouraged and this procedure appears to be gaining more widespread acceptance for such use.