The mitotic index of the Langhans' cells of the normal human placenta has been evaluated from the 6th week of pregnancy until the term. The index is very high from the 6th to the 10th week, and afterwards it decreases quickly. However, some dividing cells are present also in placentas of 30-32 weeks. The high number of mitotic cells is considered to be related to the increase of the syncytial layer more than to the HCG production. This latter would be independent of the proliferative activity of the Langhans' cells and is probably affected by some humoral factors.