Abstract THERE is little information available regarding the numbers of reticulocytes present in the blood of chickens of different ages. Orten (1934) concluded from a survey of the literature to 1934 that nearly all of the nucleated erythrocytes of birds, fish and reptiles show the reticulum. However, this statement does not seem to be borne out in the case of birds by the data reported by some other investigators. Vaughan et al. (1930) found that pigeons normally have reticulocyte counts below 12 percent. Wills (1932) showed that successive reticulocyte counts in pigeons may fluctuate considerably over a range of from less than 10 percent to as high as 60 percent in response to dietary modifications which appear to have involved either induction or relief of anaemia. Peabody and Neale (1933) used reticulocyte levels in the pigeon to measure the hematopoietic response to liver extracts and reported that the birds which had …