1. Daily eosinophil counts were obtained on a group of 10 normal individuals. This control group showed a moderate daily fluctuation in the count, with only 1 exception.
2. Daily eosinophil counts were obtained on 16 chronically ill or convalescent patients to whom A-Methopterin was administered. The eosinophil curves of 10 of these patients showed a fall of over 50 per cent at or prior to A-Methopterin toxicity. In 1 case, the count increased then decreased at toxicity to the initial level. In 3 cases definite evidence of toxicity was not demonstrable and the eosinophil level did not decrease.
3. Six patients showed a fall in their eosinophil counts before clinical evidence of A-Methopterin toxicity. Three patients showed a fall in their eosinophil counts of 75 per cent at the time of A-Methopterin toxicity.
4. Twelve of 16 cases (75 per cent), to whom A-Methopterin was administered showed definite falls in the curves of daily eosinophil counts. That cortisone and the adrenocorticotropic hormone do likewise suggests, but is not evidence, that the mechanisms are related.