Although the careful selection of shell-forming polymers for the construction of nanoparticles is an obvious parameter to consider for shielding of core materials and their payloads, providing for prolonged circulation in vivo by limiting uptake by the immune organs, and thus, allowing accumulation at the target sites, the immunotoxicities that such shielding layers elicit is often overlooked. For instance, we have previously performed rigorous in vitro and in vivo comparisons between two sets of nanoparticles coated with either non-ionic poly(ethylene glycol) (PEG) or zwitterionic poly(carboxybetaine) (PCB), but only now report the immunotoxicity and anti-biofouling properties of both polymers, as homopolymers or nanoparticle-decorating shell, in comparison to the uncoated nanoparticles, and Cremophor-EL, a well-known low molecular weight surfactant used for formulation of several drugs. It was found that both PEG and PCB polymers could induce the expression of cytokines in vitro and in vivo, with PCB being more immunotoxic than PEG, which corroborates the in vivo pharmacokinetics and biodistribution profiles of the two sets of nanoparticles. This is the first study to report on the ability of PEG, the most commonly utilized polymer to coat nanomaterials, and PCB, an emerging zwitterionic anti-biofouling polymer, to induce the secretion of cytokines and be of potential immunotoxicity. Furthermore, we report here on the possible use of immunotoxicity assays to partially predict in vivo pharmacokinetics and biodistribution of nanomaterials.