The influence of the microcarrier type on the performance of a controlled release process used to produce a recombinant glycosyl-phosphatidylinositol anchored protein was investigated. Chinese hamster ovary (CHO) cells expressing the human melanoma tumor antigen (p97) were cultured in 10% serum on Cultispher-GH porous microcarriers and then, for protein production, maintained in 2% serum. Cells were harvested every 48 h and p97 was recovered at 90 mu g/mL and 40% purity. Harvested p97 concentrations were increased by harvesting from spheroid (241 mu g/mL) and smaller porous microcarrier, Cultispher-G (167 mu g/mL) cultures. The low total cell specific p97 production of cells cultured on Cultispher-GH was due to necrosis of cells within the beads, decreased p97 expression of the immobilized cells, dilution by the liquid (up to 40% volume) associated with settled beads, and incomplete recovery of p97 from within the beads. Cells cultured on solid microcarriers, Cytodex-1, had the highest cell viability and cell specific p97 production. It is recommended that a two-stage cyclic harvesting process of cells cultured on small Cultispher-G or on Cytodex-1 beads would minimize protein loss and maximize cell specific protein recovery.