Naltrexone (NTX), an opiate antagonist, had an inhibitory effect on the growth of S20 Y neuroblastoma (NB) in A/Jax mice. Daily injections of 0.1 mg/kg NTX resulted in a 69% tumor take, 70% delay in time prior to tumor appearance, and a 60% increase in median survival time. Inoculation of NB in control mice resulted in 100% tumor take within 15 days. The pattern and incidence of metastases of NTX and control mice were similar. These results show that NTX has antineoplastic activity, and suggests a role for the endogenous opioid system in neuro-oncogenic events.