Bovine chromaffin cells were microencapsulated within alginate-polylysine-alginate (APA) membranes. Microencapsulated bovine chromaffin cells as well as unencapsulated cells and empty microcapsules were grafted into the brain of hemiparkinsonian rats with 6-hydroxydopamine (6-OHDA) lesions. Apomorphine-induced rotational behavior of the host animals and the survival of the grafted chromaffin cells were examined after transplantation. The animals receiving microencapsulated bovine chromaffin cells showed a significant decrease (17.6--35.6%) in apomorphine-induced rotation 1 week postimplantation that remained stable for the 10 month test period. Fluorescent histochemistry further revealed that microencapsulation increased the chromaffin cell survival with only a minimum host reaction for up to 10 months posttransplantation while the survival of free, unencapsulated chromaffin cells was only modest and was accompanied by a large inflammatory response. The reduction of apomorphine-induced rotations was correlated with the survival of bovine chromaffin cells in the host brain. The data indicate that encapsulation of bovine chromaffin cells in APA membranes reduces the host immune response to the xenograft and prolongs the viability of the grafted cells.