Alginate purification has been shown to decrease the host immune response to implanted alginate-based microcapsules, but the direct effect of contaminants on islet cell survival remains unknown. Wistar rat islets were immobilized in calcium alginate beads made with crude vs. purified alginate and then incubated in CMRL culture medium. Islet survival was evaluated at 1, 4, 7, 14 and 27 days post-encapsulation. Islet viability was investigated using a dual staining assay (propidium iodide and orange acridine). The islet cell necrosis and the proportion of apoptotic cells were quantified under optical microscopy and with a TUNEL assay, respectively. Islets immobilized in purified alginate were more viable, and had fewer necrotic centers, a smaller area of central necrosis and a lower number of apoptotic cells. At day 14 and 27 post-encapsulation, respectively, 48% and 23% of islets were viable with purified alginate vs. 18% and 8% with crude alginate (p<0.05). At day 14, the surface area of central necrosis and the number of necrotic islets were more important with the impure alginate (65% vs. 45% and 73% vs. 53%, respectively; p<0.05). We conclude that alginate purification improves the survival of islets that are immobilized in alginate-based microcapsules. These findings indicate that caution should be taken in the interpretation of in vivo experiments, as the results could be explained by either a direct effect on islet survival or a modification of the host reaction, or both. Moreover, it suggests that the effect on islet viability should be assessed during the development of biomaterials for cell encapsulation.