For establishment of a reproducible model of human neuroblastoma, 2 to 5 million of established neuroblastoma cell lines (SK-N-SH, SK-N-MC) were injected s.c. or i.p. into 20 nu/nu mice of a predominantly Swiss back-ground. Following latency periods of 8 to 21 days, tumors developed at the injection site and grew to 4-ml volumes within 3 weeks. Histologically, the tumors resembled the original metastases from which the tumors were derived; however, the SK-N-SH appeared to have evidence of morphological differentiation. When compared to monolayer culture, the heterotransplanted SK-N-SH tumor had decreased dopamine-beta-hydroxylase activity and elevated cyclic adenosine 3':5'-monophosphate phosphodiesterase activity. Activity of cyclic adenosine 3':5'-monophosphate phosphodiesterase in the transplanted SK-N-MC tumor was not appreciably different from the activity in the cultured cells. Serum dopamine-beta-hydroxylase levels in the mice bearing SK-N-SH tumor increased threefold. The SK-N-MC cultured cells lacked dopamine-beta-hydroxylase and did not alter existing serum levels in the SK-N-MC tumor-bearing mice. 67Ga injected i.v. was found to localize in the tumor after 24 hr. Human neuroblastoma in the nude mouse can be a reproducible and informative model for tumor pharmacology, screening, radionuclides, tumor localization and imaging, and investigating morphological differentiation.