Four sequential bone-marrow aspirations from the ipsilateral tibia and iliac crest of New Zealand White rabbits, aged 4 months or 1, 2, or 3 years, were assayed in vitro and in vivo for their chondroosteogenic potential. Nonhematopoietic cells from the samples of bone marrow were isolated and expanded in culture: their colony-forming efficiency was determined, and second-passage marrow-derived cells, referred to as mesenchymal progenitor cells, were loaded into porous calcium-phosphate ceramic cubes as carrier vehicles for an in vivo cartilage and bone-formation assay. The cubes were placed subcutaneously in nude BALB/C mice for 3 and 6 weeks. On histological examination, the cubes were scored for the presence of bone and cartilage in their pores, and average values for age groups and location were compared. At aspiration, the samples from the iliac crest exhibited an overall reduction in cell concentration with increasing age, and at the first harvest time, they showed a decrease in colony-forming efficiency and cube score with increasing age. This study demonstrated that repeated bone-marrow aspirations may be performed and may have an enhancing effect on the osteochondral progenitor cells of older animals.