In this study, the ultrastructure of the human placental 'nematosome' or 'glomerular body' is described. It is usually found as electron-dense aggregates forming an inclusion 0.3 to 0.5 micron in diameter and of variable length, and occurs within cytotrophoblast cells in different configurations which include spirals, concentric rings, parallel arrays, random coils and paracrystalline structures. Analysis of random electron micrographs of placentae from normal and complicated pregnancies showed that the nematosome is present in low frequencies in the first and second trimesters but is more commonly found at full term; their incidence did not differ markedly from normal in any of the pathological conditions studied, ranging from 10.4 to 21.9 per cent, and their frequency suggests that there may be several nematosomes per cell. The relationship of the nematosome to the centriole is discussed, and it is suggested that morphometric analysis may elucidate the three-dimensional structure of nematosomes, their distribution within cells and their relationship to the plasma and other membranes.