將一個物種的組織移植到另一個物種體內，例如，從豬到人。和同種移植不同的是，異種之間存在很大差異，從而使得這種移植成功的可能性很小。負責組織排斥的免役系統將很容易地識別出外來組織并強烈排斥它。既然動物可以為移植提供無盡的來源，異種移植一直是人們夢寐以求的事。豬雖然看上去和人有著很大的差異，卻有著相似的器官結構，因而成為該領域內研究的焦點；猴子是另一類有吸引力的種群。

A tissue transplant from one species to another, e.g. from pig to human. Because of the greater differences between species, as opposed to within a species, these transplants have the least chance of working. The immune system, which is responsible for tissue rejection, will easily recognize the tissue as foreign and will reject it vigorously. Thus xenograft, or xenotransplantation is a sort of holy grail for transplantation, since animals would provide an endless supply of organs for transplantation. Pigs, although seemingly very different from humans, have similar organ organization and so remain a focus for research in this area; monkeys are another attractive group.