Given the presence of several concerning compounds and those unidentified in the consumergrade PCL extracts, we hypothesized that the acute toxicity of the consumer-grade PCL to developing zebrafish embryos could be due to these impurities and additives. In a follow-up experiment, we tested biomedical grade PCL for its toxicity to zebrafish embryos in an effort to reduce any confounding effects from leachable chemical additives. Because residual tin catalysts can reduce the biocompatibility of PCL-based biomedical implants, biomedical-grade PCL is purified to reduce residual tin below 50 ppm. Purification presumably also removes residual catalysts and other non-intentionally added substances. If the acute toxicity persisted for this material, it would suggest that components intrinsic to the polymer, i.e., nanoplastics and oligomers, were more likely the cause of toxicity than chemical additives. To test this, we evaluated the toxicity of a commercially available biomedical-grade PCL certified to have residual tin content of 18 ppm, residual monomer content ≤0.5%, and other elemental impurities ≤10 ppm by the United States Pharmacopeial method 232. Dosed at the greatest plastic concentration for consumer-grade PCL tested (~60 mg/mL), we observed an ~80% mortality of zebrafish embryos within 3 dpf when directly exposed to biomedical-grade PCL (Figure ). These results indicated that the observed acute toxicity for PCL was unlikely to be from a chemical additive, residual catalyst, or non-intentionally added substance and suggest that polymer breakdown products (i.e., nanoplastics and oligomers) were the source of the toxicity.