Biocompatibility of biomaterials and biomaterial-based medical devices is a critical issue for the long-term function on multiple therapeutic systems. In the past few years, there has been an increasing interest in producing more biocompatible biomaterials and in developing novel assays to analyze the quality of the products. In this study, a battery of in vitro techniques to assess the biocompatibility of alginates with different compositions and purities and alginate-based microcapsules is presented. Study of the protein and polyphenol content of the alginates revealed clear differences between the nonpurified and the purified alginates. A similar behavior was observed when the mitogenic activity and the tumor necrosis factor-alpha secretion induced by the alginates were assessed. Interestingly, when the latter two techniques were adapted to evaluate the different alginate microcapsules, a correlation with the results obtained for the alginate samples was observed. These results reinforce the idea of using the full battery of assays here reported to screen alginates and alginate-based microcapsules before implantation.