Erythrocyte counts made on 18 roosters, 18 hens, 18 capons, 12 sinistrally ovariec-tomized poulards, and 3 bilaterally ovariectomized poulards resulted in the following avgs., respectively: 3.60, 2.70, 2.51, 2.78, and 2.59 millions per c. mm. 24 daily injns. of from 1.00 to 3.75 mg. of testosterone propionate raised the avg. of 6 capons to 3.30, 3 sinistrally ovariectomized poulards to 3.77, and 3 bilaterally ovariectomized poulards to 3.34. . There appeared no correlation between the size of the dose and the degree of increase. 21 days after inj. were discontinued the number of erythrocytes had fallen, and by 45 days had reached the level of uninjected capons and poulards. An increase in the red cell counts of juvenile [male][male]and [female][female]was brought about by 24-28 daily injns. of 140 I.U. P.M.S. gonadotrophin, begun the 4th day after hatching. Eleven capons, 3 bilaterally ovariectomized poulards, and 2 roosters were given daily doses of 0.33 to 1.50 mg. of a-estradiol benzoate or 1.00 to 5.00 mgm. of diethylstilbestrol for 15 to 29 days. In all cases the number of erythrocytes was reduced and clotting time was prolonged. Counts on 36 intersexual [male][male](feminized by estrogen inj. during incubation) were at the $ level, but there appeared no correlation between degree of feminization of the plumage and the degree of erythrocyte reduction, since avg. count (2.83) of 7 birds essentially masculine in appearance was approx. the same as the avg. count (2.73) of the 6 most feminine birds. From these results it would seem that sex hormones influence the number of crythrocytes in the blood of brown Leghorn fowl, androgen causing an increase and estrogen a decrease.