Erythropoietin (EPO) and its cell surface receptor (EPOR) are essential for erythropoiesis; can modulate non-erythroid target tissues; and have been reported to affect the progression of certain cancers. Basic studies of EPOR expression and trafficking, however, have been hindered by low-level EPOR occurrence, and the limited specificity of anti-EPOR antibodies. Consequently, these aspects of EPOR biology are not well defined, nor are actions of polycythemia- associated mutated EPOR alleles. Using novel rabbit monoclonal antibodies to intracellular, PY- activated and extracellular EPOR domains, the following properties of the endogenous hEPOR in erythroid progenitors first are unambiguously defined. 1) High- Mr EPOR forms become obviously expressed only when EPO is limited. 2) EPOR-68K plus -70K species sequentially accumulate, and EPOR-70K comprises an apparent cell surface EPOR population. 3) Brefeldin A, N-glycanase and associated analyses point to EPOR-68K as a core-glycosylated intracellular EPOR pool (of modest size). 4) In contrast to recent reports, EPOR inward trafficking is shown (in UT7epo cells, and primary proerythroblasts) to be sharply ligand-dependent. Beyond this, when C-terminal truncated hEPOR-T mutant alleles as harbored by polycythemia patients are co-expressed with the wild-type EPOR in EPO-dependent erythroid progenitors, several specific events become altered. First, EPOR-T alleles are persistently activated upon EPO- challenge, yet are also subject to apparent turn-over (to low-Mr EPOR products). Furthermore, during exponential cell growth EPOR-T species become both over-represented, and hyper-activated. Interestingly, EPOR-T expression also results in an EPO dose-dependent loss of endogenous wild-type EPOR's (and, therefore, a squelching of EPOR C-terminal- mediated negative feedback effects). New knowledge concerning regulated EPOR expression and trafficking therefore is provided, together with new insight into mechanisms via which mutated EPOR-T polycythemia alleles dysregulate the erythron. Notably, specific new tools also are characterized for studies of EPOR expression, activation, action and metabolism.