Newcastle Disease Virus (NDV), an agent with interesting immune stimulatory and anti-tumor activity, was investigated for its capacity to activate anti-tumor activity in murine macrophages in vitro and in vivo. Direct macrophage activation was seen under a variety of experimental conditions using two different strains of NDV, different sources of macrophages (spleen and peritoneum) and different strains of mice (DBA/2, C57BL/6, 615). Various macrophage enzymes (ADA, iNOS, lysozyme, acid phosphatase) became upregulated and anti-tumor effector molecules such as nitric oxide (NO) and TNF-alpha were found in the supernatant. NDV activated macrophages performed anti-tumor activity in vitro such as anti-tumor cytostasis and anti-tumor cytotoxicity. The cytotoxic anti-tumor activity was broad and active against all tumor lines tested including mammary carcinoma, lung carcinoma, mastocytoma and immune escape variants (lymphoma). Macrophage activation via BCG/LPS also caused a broad range anti-tumor cytotoxic activity while activation via mixed lymphocyte culture conditioned medium had restricted anti-tumor activity. Anti-tumor activity of NDV activated macrophages could be transfered in vivo. Transfer of macrophages which had not been appropriately activated exerted either no effect or a tumor growth augmenting effect. Repeated intravenous transfer of NDV activated macrophages exerted a significant suppressive effect on pulmonary metastases in a mammary carcinoma tumor model as well as in a lung carcinoma model. Taken together these results demonstrate that NDV can strongly activate macrophages to perform anti-tumor activities in vitro and in vivo.