This is the first study to report on the biocompatible and immunogenic properties of one-pot synthesised gold and silver nanoparticles (Au@UI and Ag@UI) using the macroalgae Ulva intestinalis (UI). The UI aqueous extract, Au@UI, and Ag@UI were obtained under sterile conditions and fully characterized by UV-vis spectroscopy, TEM, HRTEM, STEM and FTIR spectroscopy. Moreover, for the first time, the composition of carbohydrates in the UI extract has been reported along with the changes observed after nanoparticle synthesis by size exclusion chromatography, in order to investigate their possible role in the biosynthetic process. This study suggested that the polysaccharide fraction of the extract is involved in the formation and stabilization of the nanoparticles. The potential toxicity of the samples was evaluated using different cell lines and the hemocompatibility was tested in mouse erythrocytes. In addition …