Most repons of treatment of lymphosarcoma (LSA) in domestic animals have involved canine and feline LSA. The disease in cats differs from that in dogs in that feline LSA is caused by a contagious retrovirus. Controversy has existed as to whether feline leukemia virus (FeLV) positive cats with LSA should be treated because of their danger to other cats, or because of the controversial question of a human health hazard. There has been no proven danger to people, and 30% of cats with LSA are FeLV negative. Some owners elect chemotherapy for their cats regardless of potential risks. As a result of this increasing awareness of pet owners to the potential benefits to be gained by chemotherapy, the practicing veterinarian must be able to skillfully administer chemotherapy to pets who have developed cancer.