Seventy-five human embryos, at stages ranging from XII to XVIII of Streeter's horizon, four to six weeks postconceptional age, were obtained from healthy women by curettage. Limbs were cultured in nutrient agar medium for four to 18 days. Excepting a few cases which resulted in total necrosis, differentiation of mesenchyme into cartilage or bone primordia and other supporting tissues took place similarly to in vivo, though it occurred more slowly compared to in vivo. Such developmental retardation was milder in the lower limbs than in the upper limbs. In horizons XIV through XVIII, the limbs cultured for longer period than four days showed developmental dissociation of mesenchymal tissues, i.e., the maturation of cartilage advanced more than did the development of other tissues of the limb. The time lapse between the curettage and the culture ranged from 1.5 hours to seven days, but this had no close relationship with the occurrence of degenerative changes. Comparing the data obtained here with the corresponding results of a similar study in mice, it was concluded that the differentiation of the human limb occurs far slower than the mouse limb and shows some different patterns. These results suggest that it is possible to study in vitro comparison of effects of some teratogens on human and mouse limbs.