Measurement of plasma ERFE after EPO injection and in thalassemic mice. ERFE concentration was measured in plasma samples of EPO-treated WT mice (A) and littermate WT and Th3/+ mice at 3, 6, and 12 weeks of age (B). (A) ERFE levels in WT mice gradually and significantly increased between 4 and 15 hours after administration of EPO (200 U), reached maximum at 15 and 24 hours, and decreased again by 48 hours. Plasma ERFE concentration was elevated into a similar range in Th3/+ mice (B) at 3, 6, and 12 weeks of age compared with WT mice (P < .001), in which ERFE levels were lower than the limit of detection (100 pg/mL). Data shown are means ± SEM and were compared for each point to values for control mice at t = 0 (n = 4 mice per group and condition) in EPO-treated mice and for each age group between WT and Th3/+ mice (n = 6 to 13 per group) by 2-tailed Student t-test. ***P < .001, *P < .05.