Feline leukemia virus-induced erythroid aplasia: in vitro hemopoietic culture studies. 
Colony forming unit (CFU) assays were developed for feline granulocyte-macrophage (CFUGM), early erythroid (day 2 CFUE), and late erythroid (day 7 CFUE) colonies in methylcellulose medium.  Feline CFUGM and both day 2 and day 7 CFUE were enhanced by feline macrophage conditioned medium and late CFUE often were intimately associated with macrophages.  Kittens were inoculated with the Kawakami-Theilen (KT) strain of feline leukemia virus (FeLV) and sequential changes in marrow CFU determined.  Erythroid aplasia, characterized by progressive non-regenerative anemia, lymphopenia, and a profound decrease in early and late CFUE but not CFUGM was induced by 3 to 5 weeks after FeLV-KT inoculation.  The susceptibility of kittens to FeVL-induced erythroid aplasia was strongly age-related; neonatal kittens were most sensitive and substantial natural resistance developed by 4 weeks of age.  The results demonstrate that FeLV-KT infection induced a rapid and selective suppression of erythroid progenitor cells and represents a suitable model of experimentally-induced acquired erythroid aplasia.