Aims: The aims of this work were to investigate the effects of sera on B.similar to anthracis Sterne germination and growth. Sera examined included human, monkey and rabbit sera, as well as sera from eight other species. Methods and Results: Standard dilution plate assay (with and without heat kill) was used as a measure of germination, and spectroscopy was used to measure growth. In addition, a Coulter Counter particle counter was used to monitor germination and growth based on bacterial size. Spores germinated best in foetal bovine and monkey sera, moderately with human sera and showed limited germination in the presence of rabbit or rat sera. Vegetative bacteria grew best in foetal bovine sera and moderately in rabbit sera. Human and monkey sera supported little growth of vegetative bacteria. Conclusion: The data suggested sera can have a significant impact on germination and growth of Sterne bacteria. Significance and Impact of the Study: These data should be considered when conducting in vitro cell culture studies and may aid in interpreting in vivo infection studies.