Using a colorimetric method, values for serum iron and total iron binding capacity were determined in 33 feral Soay sheep, 32 domesticated Soay sheep, and nine Welsh Mountain sheep. The feral flock showed a higher mean total iron binding capacity than the domesticated Soay flock and the feral males showed a mean serum iron level higher than that of the males of the domesticated Soay flock. Within the feral flock the adult males showed a serum iron level higher than that of the adult females. Within the domesticated Soay flock the serum iron level of the male lambs was higher than both that of the female lambs and that of the male adults. The serum iron level and total iron-binding capacity of the Welsh Mountain flock were lower than those of the Soay sheep. Results of other workers have been listed.