Patients with large burns suffer from anemia of critical illness. Administration of exogenous erythropoietin is ineffective, and transfusion remains the only effective treatment. We have previously shown that erythroid precursors are decreased 1 week after burn in an animal model. Therefore, we have used a two-phase liquid culture system to quantify peripheral blood mononuclear cell (PBMC) compartment-derived erythroid progenitors (EPs) in burn patients. Institutional review board approval and informed consent were obtained. Blood samples were collected at 1 to 30 days after burn, with a mean TBSA of 37.7 ± 15.8% (n = 10; 90% men; age, 46.0 ± 18 years). Four healthy volunteers served as controls. PBMCs were isolated by Ficoll-Hypaque density-gradient centrifugation and were placed in serum-free expansion medium containing cyclosporine A (1 ng/ml), granulocyte macrophage colony-stimulating factor (20 ng/ml), stem cell factor (30 ng/ml), and interleukin-3 (5 ng/ml; phase I). On day 7, cells were reseeded in serum-free expansion medium containing erythropoietin (1 U/ml), holotransferrin (0.3 mg/ml), and stem cell factor (10 ng/ml; phase II). Aliquots from the phase II culture system on day 6 were incubated with anti-CD71, CD235a, and CD36. EPs (CD71 CD36) and erythroblast subpopulations (colony-forming unit erythroids, Proerythroblasts, and intermediate erythroblasts) were identified based on the expressions of CD71 and CD235a by flow cytometry, calculated per million expanded cells, and expressed as a percentage of controls. Total EPs were significantly decreased by days 28 to 31 after the burn (19%; P < .05). Among the erythroblast subpopulations, colony-forming unit erythroids (11%; P < .004) and proerythroblasts (24%; P < .05), were decreased significantly by days 28 to 31 after the burn. PBMCs of burn patients can be used to study impaired erythropoiesis and anemia of critical illness.