INTRODUCTION: The unicellular trophoblast epithelium of all ruminants so far investigated contains 15-20% binucleate cells with numerous secretory granules. Electron microscope (EM) studies of the domesticated cow, ewe, goat and deer species have established that these BNC migrate out of the trophoblast epithelium to fuse with the apposed maternal uterine epithelial cells or derivative to form fetomaternal tissue throughout pregnancy. However there is one careful EM study of the trophoblast of a wild ruminant, the White-tail deer, which found the usual number of BNC but no evidence of any migration or fusion. Since there are up to 200 species of wild ruminants, it was important to establish whether there really are two possible scenarios for BNC function. MATERIALS AND METHODS: This paper reports a light microscope (LM) immunocytochemical study of cell dynamics in ruminant placentas using 1-2 mμ deresinated sections. RESULTS: The results clearly demonstrate that the White-tail deer and all of the other 15 (see Table 1) randomly selected wild ruminants show the same BNC migration and fusion pattern. DISCUSSION: These results suggest that this remarkable cellular behaviour is fundamental to the ruminant evolutionary success.