The types of anemia associated with natural and experimental feline leukemia virus (FeLV) infection in cats were investigated. In one experiment, 10 kittens were inoculated neonatally with Jarrett FeLV-1, an isolate of subgroup A; 6 developed anemia a few weeks later. This anemia was characterized by macrocytosis, normoblastosis, increased erythropoiesis in the bone marrow, and extramedullary hematopoiesis in the spleen. Anemia was transient and nonfatal and occurred before the onset of lympoid malignancy. The same type of anemia was also seen in 9 of 24 kittens inoculated with Jarrett FeLV-9 of subgroups A and B. A different form of anemai occurred in another experiment in which 10 kittens were inoculated with FeLV-C of subgroup C only. All 10 kittens developed a profound aplastic or erythroblastopenic anemia in which the bone marrow became depleted of erythroid tissue; all kittens died within 16 weeks, most as a direct result of anemia. In an experiment in which kittens were inoculated with FeLV-B of subgroup B only, no kitten showed anemia. Cats with naturally acquired, nonleukemic lymphosarcoma were also studied. Of 33 lymphosarcomas in which myelophthisis was excluded as a cause, 54% of the affected cats had anemia, the features of which were consistent with hemolytic origin. When virus could be grown from these lymphosarcomas, it was of subgroup A alone or a combination of A and B. With one exception, anemic cats had low or negative titers to feline oncornavirus-associated cell membrane antigens. Until more isolates have been tested, it is not known if the various hematologic changes reflected differences in the pathogenic effects of the subgroups of the virus or of types of strains within them.