Stage IV-S neuroblastoma, characterized by a primary tumor plus disseminated tumors in liver, skin and bone marrow, has a favorable clinical prognosis when compared to metastatic Stage IV neuroblastoma. This favorable outcome also characterized mice receiving tumor transplants to these "IV-S" sites. We report the testing of the hypothesis that enhanced anti-tumor immunity in "IV-S" site neuroblastoma recipients explains this improved survival. A million murine C1300 neuroblastoma cells were inoculated into 256 A/J mice to either "IV-S" sites of skin, liver, peritoneal cavity, or to the disseminated stage "IV" sites of subcutaneous tissue, muscle, kidney and lung. After 21 and 28 days of tumor growth, spleen cells from tumor bearing mice were harvested and analyzed by a 51 Cr release lymphocytotoxicity assay. Cytotoxic T cell activity was consistently higher at day 28 than day 21. In the liver and in the peritoneal cavity, cytotoxic T cell activity was higher than in other organs, and at day 28 these values were significantly higher than Stage "IV" sites. On the other hand, skin is not a immunologically privileged site in vivo study.