81 primary pediatric tumors and 4 tumor lines were heterotransplanted into nude mice with an overall success rate of 38.3%. There was variability in success between tumor types. Bone sarcomas were highly successful while brain, lymphoid, and benign tumors in general did poorly. With increasing passage lag times decreased but actual growth rates in general did not change. Results suggested that tumors obtained prior to therapy which grew in nude mice were more likely to recur in the patient. During the observation period 7 spontaneous mouse tumors developed, distinguished from human tumors by histology, cytogenetics, and isoenzyme studies.