In this article we report inhalation effects of nitrogen dioxide (NO2) and ozone (O3) mixture as well as O3 alone on blood-borne cancer cell colonization of lungs. The findings are discussed in light of our earlier studies with NO2 exposure alone. In all of these studies the mouse B16 melanoma model was used. Animals were exposed to ambient concentrations of pollutants before melanoma-cell infusion. The results have indicated that inhalation of NO2 played a significant role in facilitation of blood-borne cancer cell spread, while O3 inhalation did not. With respect to mechanisms involved, the role of natural immunity was investigated and its was postulated that nitrogen dioxide may affect cells of the immune system and may in part account for the results. These findings may have direct bearing on dissemination of human cancer cells, since many cancer patients have circulating cancer cells and are exposed daily to noxious air pollutants. Most importantly, this effect may be preventable by reducing air pollution in urban areas.