Nocardia lysozyme digest (NLD), a particulate fraction from Nocardia opaca, is able to induce antitumor activity to SaL-1 tumor cells (lung sarcoma) in Balb/c mice. In mice immunized with NLD inhibition of tumor growth and prolonged survival of tumor bearing animals was observed. Macrophages isolated from peritoneal cavity and stimulated with NLD release a few arachidonic acid metabolites, mostly PGE 2. Macrophages from tumor bearing mice are more sensitive to Nocardia antigens than normal. Both in vitro and in vivo experiments have documented that Nocardia is an active immunomodulator.