Reticulocytes are the immature form in which erythrocytes are released into the circulation from the bone marrow. They mature into erythrocytes within about one day of release.

Reticulocytes cannot be easily distinguished in routine blood smears, but when fresh blood is incubated with the basic dye brilliant cresyl blue, a blue-stained reticular precipitate highlights any reticulocytes present. This is due to the interaction of the dye with ribosomal RNA still remaining in the immature cells. Here the reticulocytes are seen as RBCs possessing dark filamentous strands within the cells.