This is the report of an international symposium held at Sukhumi, USSR, in May 1965. Thirty-five contributors from all over the world (including both sides of the iron curtain) delve into experimental methodology and immunology of viruses and tumors. The subject of experimental cancer prophylaxis through immunologic methods is debated. This appears to be the most comprehensive discussion of the subject which has appeared to date. Spirited dialogue follow each formal paper.