The hollow fiber assay is a unique in vivo model that allows simultaneous evaluation of up to 6 different cell lines in 2 physiological separate compartments. It was developed by Hollingshead et al. (1) as a preliminary rapid screen for assessing novel putative chemotherapeutic compounds prior to their evaluation in the mouse xenograft model. The hollow fiber model has a shorter evaluation time and a reduced compound requirement compared to traditional xenograft models. The model allows for the effective pairing of a novel compound with the appropriate cell line by its capacity to utilize multiple cell lines.