The effect of mouse age on the in vitro migration of hemopoietic precursor cells from bone marrow of C57BL/6 to thymic supernatant from newborn mice was studied to determine whether a reduction in the migratory readiness or number of precursor cells might at lest partly explain thymic involution due to aging. The percentage of bone marrow cells migrating to the thymic supernatants increased to age of 7 weeks, and then decreased progressively. Thymus weight underwent almost exactly the same evolution. The decline of the population of migration-ready T-precursor in adult mice may this explain age-related thymic involution.