The maternal organism plays a highly important role in transplacental carcinogenesis, since for carcinogens in the bloodstream of the mother to reach the fetus, they must cross several barriers, the first of which is the placenta. Some types of compounds require metabolic activation in the maternal organism, in the fetus and even sometimes in the placenta. Thus, four main pathways can be hypothesized by which substances exert a carcinogenic effect on the fetus. Most carcinogens can cross the placenta; data confirm that this process consists of simple diffusion or - in the case of high doses - facilitated diffusion. That carcinogens may be detoxified in the maternal organism is confirmed by experiments on activation of enzyme systems and on caesarean deliveries. Species and strain specificities are characteristic of transplacental carcinogenesis and are manifested in organotropism. Organotropism in transplacental carcinogenesis is determined by genetic predisposition, cell differentiation and proliferative activity in the target tissues. For indirect carcinogens, the level of metabolizing enzymes is also important.