The in vivo hollow fiber assay was developed at the National Cancer Institute (NCI) to help bridge the gap between in vitro cell-based assays and human tumor models propagated in immunodeficient mice. The goal was to develop an intermediate assay that could help predict which compounds found active in the 60-cell line panel would be active in a subsequent xenograft system. This was necessary due to the high cost of the traditional xenograft assay in terms of number of animals required, time for assay completion, and financial commitment necessary. To address this problem, investigators of the NCI Developmental Therapeutics Program designed a method of propagating human cancer cells in inert hollow fibers with pores small enough to retain the cancer cells but large enough to permit entry of potential chemotherapeutic drugs, including large proteins and other important substances. Fibers containing proliferating cancer cells are transplanted into the peritoneum or under the skin, the host mice are treated with a test agent, and the fibers are subsequently retrieved for analysis of viable cell mass. The assay has been successful in helping investigators from around the world, including our own research group, prioritize compounds active in vitro for further testing in the traditional xenograft system.