Nude mice inoculated with human colon cancer (HT-29) and receiving 0.1 mg/kg naltrexone (NTX) beginning immediately after tumor cell injection exhibited a marked retardation in tumorigenicity. This dosage of NTX, which blocked opioid receptors for 6-8 h/day, resulted in a delay of 2.4-fold in tumor appearance compared to control subjects. At the time (10 days) when all control mice had tumors, 80% of the mice in the 0.1 mg/kg NTX group had no signs of neoplasia. Binding capacity, but not affinity, of [3H][Met5]-enkephalin was reduced 85% of control levels in tumor tissue from mice of the 0.1 NTX group. Plasma, but not tumor tissue levels of [Met5]-enkephalin were elevated (2.5-fold) in contrast to control values. These results suggest that daily intermittent opioid receptor blockade with NTX provokes the interaction of opioids and receptors in the interval following drug availability, with opioids serving to inhibit tumorigenicity of human colon cancer.