The effects of endotoxin or testosterone on the amount of transferrin 59Fe (or like protein) in the murine macrophages was investigated. The mouse peritoneal macrophages were laden with 59Fe tagged red cells following the injection of mice with either agent. After harvesting the cells, they were lysed and the transferrin iron was released with 40% trichloroacetic acid. The supernatant (extract transferrin iron) and the pellet (other iron proteins iron) were separated by centrifugation and their radioactivities counted. The results were expressed in percentage. The endotoxin group had a geometric mean of transferrin 59Fe of 0.14% compared to 0.28% for the control, p less than 0.001. The geometric mean for transferrin 59Fe of the testosterone treated group was 0.51% compared to 0.35% for the control, p less than 0.05. Therefore, the endotoxin seems to contract the transferrin pool whereas testosterone seems to expand it.