At present, the appearance of neutrophils in the peripheral blood is considered to be the earliest sign of hematopoietic regeneration after myeloablative chemoradiotherapy. We studied erythroid regeneration in 27 allogeneic and 21 autologous marrow recipients using an automated reticulocyte counter employing a flow cytometric technique. Thereby, reticulocytes can be divided into a high, moderate and low fluorescent fraction according to RNA content. Marrow ablation caused an immediate and rapid fall in highly fluorescent reticulocytes (HFR) to zero in all patients. HFR reappeared after a median time of 7 days after allogeneic and 11 days after autologous marrow grafting. This was 7 and 2 days earlier than the appearance of first neutrophils. In allogeneic marrow recipients given erythropoetin after marrow infusion the median time to reach HFR of 0.2 and 0.5 x 10(9)/l was significantly shorter than time to reach similar absolute neutrophil counts (ANC). In allogeneic marrow recipients treated with G-CSF time to rise of HFR and ANC was almost exactly the same. Thus, automated flow cytometric reticulocyte counting allows early detection of hematopoietic regeneration and may be of clinical value for the early diagnosis of inadequate marrow function after marrow transplantation and the monitoring of erythroid regeneration under cytokine administration.