1. Arteriovenous eosinophil differences across the spleen were measured in 10 dogs during the development of adrenal-mediated eosinopenia. In 7 instances the stimulus was epinephrine, in 2 ACTH, and in 1 operation alone.
2. In no case was significant net splenic uptake of eosinophils indicated. In 3 out of 7 epinephrine experiments and in 1 ACTH experiment, a significant net splenic output of eosinophils was found. For all 10 experiments averaged, the splenic output of eosinophils was significant.
3. Arteriovenous neutrophil and hematocrit differences were insignificant and were independent of simultaneous eosinophil differences.
4. It is concluded that the spleen is not a significant site of uptake on destruction of eosinophils during the development of adrenal-mediated eosinopenia. Under certain conditions, it actually extrudes eosinophils into the general circulation at a time when other factors are causing eosinophil disappearance.
5. The results suggest but do not establish the presence of a discriminatory splenic mechanism for the control of blood cells, capable of storing cells of one type while it releases others into the circulation.