Fundamental research of factors affecting cell growth in hollow-fiber bioreactors is hindered by the lack of an efficient screening tool. To address this issue, a hollow-fiber micro-bioreactor has been developed. Hollow fibers with 10 kDa molecular weight cutoffs are housed within a piece of silicone tubing. Cells are inoculated within the hollow fibers which provides a 0.2-mL culture volume. The space between the fibers and silicone tubing (5 or 16 mt) is used as a medium reservoir sufficient to feed the cells for at least 24 h. Oxygenation is provided directly through the silicone tubing so that a pump for medium recirculation is not required. As a result, many conditions can be tested simultaneously in a single incubator. Three days after inoculation at 5 x 10(6) cells/mi in the micro-bioreactor, the rho 1D4 murine hybridoma cell line reached 2.8 x 10(7) cells/mi with an antibody concentration of 0.17 mg/mL. When inoculated at 5 x 10(7) cells/mi, the cell concentration reached 1.8 x 10(8)/mL after 3 days with an antibody concentration of 1.0 mg/mL. Results from a series of experiments with the micro-bioreactor suggested that the initial growth phase of this cell line in a hollow-fiber system is dependent on the serum concentration in the medium reservoir. This prediction was tested by simultaneously inoculating two production-scale hollow-fiber bioreactor systems. The cell side of the membrane for each bioreactor contained 10% serum, but serum was added to the reservoir side of only one of the bioreactors. The cells with only basal medium in the reservoir died after a few days, while the cells with 10% serum in the medium reservoir grew rapidly. These results demonstrate that the micro-bioreactor developed here can support good cell growth and that it can be used as a research tool to predict the performance of large-scale hollow-fiber systems.