Conclusions1) Plasma obtained from phenylhydrazine treated rabbits markedly increases plasma iron turnover. The supernatant obtained after boiling this plasma retains its effect on iron turnover. 2) Bled rabbits' plasma does not affect iron turnover, but has a clear cut lowering effect on plasma iron. Dialyzed plasma of bled rabbits, however, increases plasma iron turnover, while acetone extracts of bled rabbits' plasma, significantly depress plasma iron and plasma iron turnover. 3) The fraction of Fe59 appearing in the red cells at 24 and 48 hr is increased by normal plasma, bled rabbits' plasma, phenylhydrazine treated rabbits' plasma and is not a good index of the state of erythropoiesis. 4) Analysis of the effect of anemic rabbits' plasma on iron metabolism as studied with tracer doses of Fe59 gives information on the factors controlling iron metabolism and presents a practical method for bioassay of the plasma erythropoietic factor “Hemopoietine.”