Growth of a murine hybridoma in a hollow fiber microbioreactor was poor. This corresponded to slow initial growth in the Maximizer, a pilot scale hollow fiber bioreactor system. Medium screening experiments with the microbioreactor demonstrated that the slow growth was due to dialysis of low molecular weight serum components (under about 10 kDa) from the cell side of the fibers to the basal medium on the noncell side of the fibers. Better growth can be achieved by adding serum to both sides of the fibers, but this is an expensive option. As an alternative, the microbioreactor was used to select for a population of cells that did not require serum on both sides of the fiber for optimal growth. From this population, a stable subclone was isolated using limiting dilution followed by growth assessment in microbioreactors. The subclone was cultured in the Maximizer under conditions identical to the parental cell line. The subclone reached confluency in about 9 days compared with about 16 days for the parental cell line. At confluency, the subclone produced antibody at twice the rate of the parental cell line. These results demonstrate that the microbioreactor is a useful tool for quickly isolating subclones that are better suited for growth in a hollow fiber bioreactor.