To overcome the shortage of suitable human donors for transplantation attention has recently turned to the possibility of using genetically modified pigs as a source of cells and organs. It has been suggested that such procedures might facilitate the introduction of novel retroviruses, normally resident in the pig germ line, into the human population (Stoye and Coffin, Nature Medicine 1: 1100, 1995). The consequences of such a transfer remain unclear; however, the demonstration that certain porcine cell lines express infectious retroviruses which can infect human cells (Patience et al., Nature Medicine 3: 282-286, 1997) emphasizes that there are grounds for practical concern. We have now cloned the envelope genes of the expressed viruses and are using these clones in studies of the interaction of the porcine viruses with their cellular receptors. We have also initiated studies of the inheritance and expression of human-tropic endogenous proviruses present in different pig populations. These studies reveal that at least two classes of human-tropic endogenous porcine retrovirus are widely distributed in pigs (Le Tissier et al., Nature 389: 681-681, 1997). The implications of our results for assessing the potential risk of retroviral transfer during xenotransplantation are discussed.