The mutagenic and co-mutagenic properties of harman, norharman and of some of their pharmacologically important derivatives are reviewed. These compounds do not behave as true mutagens, but rather interact, directly or indirectly with DNA, leading to various consequences. This unusual behaviour is most probably related to the particular structure of the chemical nucleus common to all /3-carbolines which confers to the different derivatives the property to interact with various macromolecules and enzymatic systems. These interactions are compiled and discussed in this review.