We examined the age-associated changes in tumor necrosis factor alpha (TNF alpha) receptor on peripheral blood monocytes and bone marrow macrophages obtained from slc:Wistar rats and slc:ICR mice. The TNF alpha receptor on monocytes and macrophages in both rats and mice showed high and low affinity sites. The Kd values of those sites gradually increased with age in both types of cells from both species. Binding capacities (receptor number) were low in 52-week-old animals. These results suggest that the TNF alpha receptor on monocytes and macrophages of these species might decrease with age in both quality and quantity.