12 new-born mice were used in this work (6 controls). One group of mice was injected with endoxan (cyclophosphamide) diluted in saline at a daily dose of 0.003 µg/g body weight for 30 days; control animals were injected with saline. After a survival period of 45 days, the animals were sacrificed and the thigh was dissected to extract the femur. It was found that the epiphyseal cartilage was greatly thinned after post-natal administration of endoxan. The cartilage cells became smaller, irregular and lost their regular palisade arrangement. The matrix was increased and stained deeply with HE. A dense layer of bone was deposited under the epiphyseal cartilage, a condition known as epiphyseodesis. This means that endoxan not only caused retardation of growth of long bones, but it also caused premature closure of the epiphysis.