We studied erythropoiesis in 31 patients with vitamin B12 deficiency by measuring serum erythropoietin (s-Epo), serum transferrin receptor (s-TfR, taken as an index of total erythroid activity), reticulocyte count, and the reticulocyte maturation index (RMI). s-Epo and s-TfR were measured with commercial immunoassays, whereas reticulocyte count and RMI were determined by flow cytometry. s-Epo (123 +/- 196 U/L) and s-TfR (4.1 +/- 2 mg/L) levels were increased in patients with vitamin B12 deficiency. The absolute reticulocyte counts were decreased (29 +/- 18 x 10(9)/L) with a relative increase in the most immature fractions (RMI: 29.6 +/- 18%). A significant negative relationship was found between s-Epo and Hb level (r = -0.65, p < 0.0001). On the average, however, s-Epo was inappropriately low for the degree of anemia, since the observed/predicted (O/P) s-Epo ratio was 0.80 +/- 0.28 in vitamin B12 deficiency vs 1.00 +/- 0.16 in a group of patients with iron deficiency anemia. It is concluded that at least a portion of patients with vitamin B12 deficiency have serum erythropoietin levels that are inappropriately low for the degree of anemia.