Promotion of cell aggregation by addition of small starch particles with and without modification with gelatin was investigated, and the influence of CHO cell aggregation by the addition of modified starch on substrate consumption and production of tissue plasminogen activator (tPA) was investigated. Addition of starch particles (approximately 10 mu m phi) covalently bonded with gelatin was very effective in forming cell aggregates ranging from 100 to 300 mu m in diameter and decreased the cell sedimentation time to one-thirtieth that of the normal time. There was almost no influence of the addition of modified starch particles on cell metabolism and tPA production. Cell aggregation makes it easy to separate cells from culture during changing of medium in a cell suspension culture without the use of special equipment, such as centrifuges, hollow fiber membranes, and spin filters.