In Part II we discussed, in general terms, the effects of carcinogenic metals upon isolated biochemical systems, bacterial systems, and tissue culture cells. In this chapter we will expand on these principles and consider their application to the screening of potentially carcinogenic metals and their compounds. Before doing this, however, we must consider some of the reasons for these studies, and also what significance these studies may have in assessing the relative risks of human cancer from exposure to the same metal or compound. Chapter 7 contains the specific methodology used in these tests.