Mechanisms underlying the age‐related decrease in the developmental capacity of thymocyte progenitors from the bone marrow (BM) were analyzed, focussing on interactions of these cells with the thymic microenvironment. We employed the experimental model in which mixtures of young and old mouse BM cells, congenic for the Thy‐1 marker, were seeded onto fetal thymus (FT) explants depleted of self lymphocytes and the levels of Thy‐1⁺ cells developing from each of the two donor types were measured. When cells from young and old BM donors were seeded simultaneously, in saturating quantities, a higher level of T cells developed from the young donors.

To find out whether there were originally more thymocyte progenitors in the young BM, we carried out the competitive colonization under limiting dilution conditions and found that the advantage of the young had diminished under these conditions, thus suggesting that the age‐related changes could not be related solely to quantitative differences. We then incubated the FT sequentially with old donor cells for 24 h, followed by young for an additional 48 h and found that the advantage of the young progenitors was eliminated. We thus established that the initial stage of colonization of the FT was important in determining the outcome of the subsequent development. The kinetics of simultaneous competition within the FT, however, revealed that the advantage of the young BM‐derived cells became significant only from day 7 in organ culture, thus suggesting that sequential divisions of these cells were at a higher level than those of the old. Recolonization of FT explants by young or old BM‐derived thymocytes obtained from the first colonization of the FT stroma showed a reduced, but still significant advantage for the young BM‐derived cells over the old. Thus, we concluded that the old BM thymocyte progenitors manifested a qualitative disadvantage which became apparent during competitive colonization of the FT.