Among the hydrogels used for microencapsulation of cells and tissues, alginate has been and will continue to be one of the most important biomaterials. A mandatory requirement for clinical immunoisolated transplantations is the reproducible production of biocompatible alginate. As shown here for alginates extracted from freshly collected algal stipes, the current assays used for validation of the quality of the alginate are not sufficient to screen for impurities arising from spores of gram-positive bacteria (and related contaminants). To assess the quality of alginate, we have developed a cell assay based on the induction of apoptosis in Jurkat cells. This assay allows in combination with the "modified mixed lymphocyte" assay a rapid and sensitive screening for any fibrosis-inducing impurities in alginate samples (even during the purification regime) as demonstrated by transplantation experiments performed in parallel with BB rats (exhibiting an elevated macrophage activity). The results clearly demonstrate that the quality of the input algal material is of key relevance for the production of transplantation-grade alginate.