Fish peptones from tuna, cod, salmon, and unspecified fish were compared with a casein one by using a new method based on Gompertz modeling of microbial growth. Cumulative results obtained from six species of bacteria, yeasts, and fungi showed that, in most cases, these fish peptones are very effective. Nevertheless, this study raised some questions about the standardization of fish raw material, the enzymatic hydrolysis of fish proteins, and the composition of the culture medium used for testing the peptones.