We aim to develop an animal-free alternative to replace experiments with fish for quantifying the impact of chemicals on fish growth at early life stages. Hundreds of thousands of fish at this developmental stage are used annually to assess the influence of chemicals on growth. Juveniles are more sensitive than adult fish, and their growth can impact their chances to survive and reproduce. The method we developed can now quantitatively predict chemical impact on fish growth based on in vitro data. The hypothesis is that reduced fish growth can be predicted based on reduced fish cell population growth scaled to fish using mechanistic models. Indeed, our pilot study (Stadnicka-Michalak et al. 2015) with two chemicals demonstrated that our approach predicts reduced growth of two fish species in excellent agreement with measured in vivo data. Based on this, we are expanding the set of chemicals using this approach in order to probe the applicability of this method for chemicals with different modes of action and physico-chemical properties.