Abstract: Hollow fiber reactors for growing microbial cells. Isotropic hollow fibers are supported in a housing inoculated with cells. Nutrient medium passing through the lumen undergoes a pressure drop resulting in radial convective flow: the nutrient medium flows outwardly from the lumen into the surrounding area adjacent the entry port and fluid surrounding the hollow fiber flows into the lumen adjacent the exit port. With the efficient distribution of nutrients and removal of product, high cell densities are achieved providing for high product yields per unit reactor volume.