Primary murine thymus cell cultures consist of thymus epithelial cells, macrophages and interdigitating dendritic cells. Results of the present study indicated that the proportion of macrophage-like cells in cultures was directly related to the age of the thymus donors through 7 weeks of age. We standardized procedures to compare primary thymic cultures derived from mice, 1 day to 115 days of age. Cultures were grown 6 to 8 days and then tested for nonspecific esterase, acid phosphatase, phagocytosis of latex beads, Fc receptors, and MAC-1 staining. Less than 10% macrophage-like cells were identified in cultures from mice 1 to 3 days-old, but the proportion increased to 50% macrophage-like cells in cultures from 7 week-old mice. Thereafter the proportion plateaued although there was considerable variation among older mice. Further studies showed that the proportions of macrophage-like cells was not affected by the number of cells initially seeded into culture, nor by the amount of serum added to the culture medium. These data suggest that macrophage/interdigitating cell precursors immigrate into the thymus for 4 to 6 weeks after birth and support the hypothesis that these precursors may be influenced to differentiate into thymus macrophages or interdigitating cells by the thymic microenvironment.