BCG, Bacillus Calmette-Guerin, has an immunostimulant capacity and antitumor activity against both experimental and human tumors. Its mechanism of action has not yet been well clarified; maybe it involves one or more immune cell populations: in fact BCG has been reported to loose its activity in immunosuppressed animals. A limiting factor for systemic use of living BCG is its high toxicity: therefore BCG-derivatives have been introduced in both the experimental and the clinical fields. For experimental use one of the most interesting of these products is BCG-cell wall: when used in laboratory animals it demonstrated an efficient dose-dependent antitumor activity and lack of toxicity. On the contrary living BCG was notably toxic and ineffective when used in high doses. An interesting approach in antineoplastic therapy is the use of BCG with tumor cells as a vaccine against micrometastases remaining after surgery, chemotherapy or radiotherapy. A vaccine containing BCG-cell wall and tumor cells (either living or x-irradiated) gave very encouraging experimental results for a possible clinical use in the treatment of tumor metastases.