Microcapsules, prepared with alginate and polylysine, were injected intraperitoneally into mice and the number of peritoneal leucocytes as well as the cells sticking to the capsule wall were counted after 4-28 days. A significant increase in host reaction was observed when the microcapsules contained an outer layer of polylysine as compared with calcium alginate beads without polylysine or microcapsules coated with an outer layer of alginate. The alginate sources influenced the host reaction significantly. After an intraperitoneal residence of 4 days, the microcapsules were mainly surrounded by macrophages. After 28 days, several cell layers surrounded the microcapsules; macrophages, multinucleate giant cells, fibroblasts and mesothelial cells.