Radioactive tracer technics were used to study erythropoietic activity in dogs infected with Ancylostoma caninum. Uninfected animals served as controls. The erythropoietic response of these dogs was evaluated by means of the rate of removal of 59Fe from the plasma and its incorporation into the red blood cells. Results revealed that in infected animals erythropoiesis was either stimulated or remained within the upper limits of normal with the exception of one animal. Under the conditions of these experiments, there was no depression of bone marrow activity. Erythropoietic activity was stimulated to a degree that compensated for the blood loss due to the hookworms. Red cells were being lost from the infected animals by a process other than aging, but none of the animals became grossly anemic. The validity of this method of studying the causes of the anemia associated with hookworm infections is discussed.