The kinetics of iron metabolism was studied in 2 healthy horses, 11 months old, and 3 infected horses two of which were 6 months and the other 21 months old. The former two of infected horses, were inoculated with virus just after the plasma-iron clearance was performed and received repetition of the same experiment at various stages of infection. These horses suffered from 2 or 3 paroxysms during the experimental period. The ⁵⁹Fe uptake of red cells in them before the first febrile attack was regarded as normal and was compared with the values obtained later. The other one was at the remission stage with a rapid recovery of red cell count. 1. Healthy horses: The average value of T 1/2 was 1.48 hours, PITR was 0.55mg/kg/day, and the rate of ⁵⁹Fe incorporated into red cells was 76%, from which the life span of red cells was calculated to be 69 days. 2. Infected horses: T 1/2 and PITR which were obtained before virus inoculation were 0.86 hour and 0.65mg/kg/day and the ⁵⁹Fe incorporation rate which was observed before the first febrile attack was 77% on the average. Values of T 1/2, PITR/kg/day and Fe_C⁵⁹ obtained according to the stages of disease were 0.67 hour, 1.08 mg and 47% at the first fever attack, and 0.66 hour, 0.84mg and 40% at the second pyrexia in one patient horse, and 0.59 hour, 0.62mg and 27% at the first fever attack, 0.82 hour, 0.76mg and 61% at the remission and 0.62 hour, 0.66mg and 57% at the second pyrexia in the other one. Shortening of T 1/2, increase of PITR and decrease of ⁵⁹Fe utilization by red cells were observed at the paroxysmal stage. At the remission stage between fever attacks, the values of T 1/2, PITR, and incorporation rate approached to the normal. In a horse in which anemia was recovering rapidly, the incorporation rate was 78%, which was normal or above normal. It is concluded, therefore, that the red cell production of the infected horse decreases around the period of pyrexia and fluctuates according to the severity or the course of disease. The function comes to a normal or more active state than normal at the remission stage in which the animal has tolerated severe anemia, showing a rapid recovery from it.

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(1) One of three types of colony form can develop from a smooth colony of S. aboratus equi when a plate culture is maintained at room temperature for an additional 72 hours after the initial 24 hours incubation at 37°C. At present, except for their marked difference in colony form and their pathogenicity for mice, these three types could not be differentiated serologically. (2) The organisms which develop on an agar-agar medium after the inoculation of spleen, obtained from autopsied mice, which received cells of these three types, were inagglutinable by the H sera (e, n, x).

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