J2E cells produce rapid, fatal erythroleukemias in vivo but still respond to erythropoietin (epo) in vitro by differentiating, proliferating and remaining viable in the absence of serum. Mutant epo receptors were introduced into these cells to determine whether they could influence the different biological responses to epo in vitro and the development of erythroleukemias. Three mutant receptors were used as cytoplasmic truncation mutants Delta 257 and Delta 321 (above box 1 and below box 2 respectively), and the cytoplasmic point mutant W282R (defective for JAK2 activation). Strikingly, the Delta 321 mutation produced a hyper-sensitive response in vitro to epo-induced differentiation and viability, but not to proliferation. In contrast with the Delta 321 receptor, the Delta 257 and W282R mutants inhibited all biological responses to epo due to impaired JAK2 phosphorylation. Significantly, erythroleukemias took almost twice as long to develop with cells containing the W282R mutation, indicating that JAK2 plays an important role in the emergence of these leukemias. These data demonstrate that mutant epo receptors dominantly altered responses of J2E cells to epo in culture and the development of erythroleukemias.