Penicillin G (Pen G) can be rapidly extracted in hollow-fiber liquid-liquid contactors using N-lauryl-N-trialkylmethylamine (Amberlite LA-2) as the extractant. n-Butylacetate is much better than decanol as a diluent for such an extraction, although decanol can give a partition coefficient four times larger. The overall mass transfer coefficient found is a function of aqueous flow on the lumen side of the fiber, and is less dependent on shell-side flow. In backextraction, the overall mass transfer coefficient determined is only one tenth that of the forward extraction, primarily because the hydrophobic hollow fibers used have a high mass transfer resistance under these conditions. The mass transfer experiments show that hollow-fiber extraction of Pen G is competitive with centrifugal extraction. The prospects for extraction of other fermentation products with hollow fibers can be estimated based on the present study.