Several strains of the Newcastle disease virus (NDV) have raised considerable interest in recent years for clinical application because of their oncolytic properties. In this study we characterized virological, immunological and anti-tumor properties of some NDV strains. The oncolytic strain MTH-68/H was the most potent interferon-alpha inducer and, after UV light inactivation, it was the only tested NDV strain which induced in human PBMC anti-tumor activity in vitro. Upon systemic application to mice bearing a virus susceptible intradermal tumor, no significant anti-tumor effects were observed with the two oncolytic strains Italian and MTH-68/H while the treatment had significant side effects as seen by loss of body weight. In contrast, when using a locoregional application model for treatment of liver metastases of luciferase transfected CT26 colon carcinoma cells, MTH-68/H showed a significant delay in tumor growth, as well as prolonged survival but no effects on body weight. Surprisingly, this CT26 murine tumor cell transfectant was resistant in vitro to virus infection and oncolysis. These results suggest: i) that locoregional application of oncolytic NDV is more effective than systemic i.v. application; and ii) that oncolytic NDV can mediate effects even against a virus-resistant tumor line. The involvement of host anti-tumor immune responses as an important mechanism in therapies based on oncolytic NDV will be discussed.