A novel gel entrapment method has been developed where macropores are created within alginate beads to provide an environment for high-density growth of mammalian cells. The method takes advantage of an interaction between poly(ethylene glycol) (PEG) and alginate to provide a network of pores within the bead for growth while the surrounding alginate matrix retains the integrity of the bead and minimizes cell leakage. Hybridomas were grown to a density approaching 10(8) cells/mL of beads in this system, while conventional alginate restricted growth to a maximum of 2 x 10(7) cells/mL of beads. In addition, cell leakage was minimal even at high cell densities, which was not the case with the conventional alginate system. Study of the conventional system determined that cell growth was limited by the alginate matrix; increasing the alginate concentrations resulted in lower final cell densities. In contrast, the PEG-alginate system permits growth in pores so the alginate matrix serves only as a structural matrix for cells. The pore size can be varied as a function of PEG concentration (10-20 wt % PEG) to provide radially defined areas for cell growth and radial diffusion pathways for nutrients/products in the adjacent alginate matrix. Because the PEG-alginate entrapment process does not require additional chemical reactions or temperature changes, the system offers a simple alternative to attain high cell densities in an immobilized bead system. As an illustration of the concept, cells entrapped in this system were grown to high density in both batch and perfusion modes for the production of monoclonal antibodies. Using the suspension batch culture as the base case, the specific monoclonal antibody production rate increased 1.6-fold for the slower growing batch-immobilized culture and 3-fold for the immobilized perfusion culture.