AbstractThe possibility that chondrocytes of the proximal fragment of Meckel's cartilage may participate in resorption of the extracellular substance of the cartilage and outlive its removal, has been investigatedin vitroandin vivo.Mandibles from 18 dayin uteromouse foetuses were cultured for 14 days on an an antibiotic‐free chemically defined medium. When tested histochemically for acid phosphatase heavy deposits of reaction product were evident in cells in areas where resorption appeared to be occurring. When maintained on medium containing 1.0 μg/ml hydrocortisone, groups of chondrocytes hypertrophied and were reactive for acid phosphatase, and this was accompanied by loss of intervening extracellular substance. These changes were intensified by increasing the oxygen tension of the environment and by further supplementing the medium with 1.0 μg/ml triiodothyronine. Chondrocytes in the vicinity of sites of resorption could incorporate3H‐proline and3H‐thymidine.In vivo, chondrocytes in the vicinity of resorbing areas in two to three daypost‐partumanimals were highly reactive for acid phosphatase, and could incorporate3H‐thymidine,3H‐proline, and3H‐uridine. These observations have been interpreted to suggest that resorption of the proximal fragment of Meckel's cartilage is not necessarily accompanied by death of the chondrocytes, and that the chondrocytes may participate in removal of the extracellular substance. Furthermore, the response of the chondrocytes of Meckel's cartilage to hydrocortisonein vitroappears to differ from the response that has been reported to occur in a number of othe