Reticulocyte release factor.

Some investigators have had conspicuous success with reticulocyte counting methods in animals as assay techniques for plasma erythropoietic hormone, but there has been some diversity in the reticulocytosis obtained by different workers. This communication reports the disparity between the marrow erythroid activity (assessed by counting colchicine-blocked mitoses) and the reticulocyte response in normal Wistar rats injected with plasma extracts. In bled rats in which mitotic activity was of comparable range to the extract-injected rats a considerable reticulocytosis resulted. Evidence is presented that the most likely explanation for the lack of reticulocytosis in the injected group is the absence of a factor necessary for the release of reticulocytes. It is emphasized that in assay methods using reticulocyte counts, weight, age, and strain of the animals may be of critical importance.