Cells resembling the smaller variants of the Warthin-Finkeldey giant cells of measles were observed in the pulmonary alveolar capillaries in a high percentage of primates and humans apparently free from pathological changes associated with measles. Cells resembling megakaryocytes were found also in the liver and spleen in both groups but only in a small minority of cases. The relative appearances and distribution of these two types of cells pointed to their both being circulating megakaryocytes, the majority being filtered off in the pulmonary alveolar capillaries, where they become compressed. The paucity of these cells in other tissues is a reflection of the small number of megakaryocytes capable of passing along the alveolar capillaries. The presence of these cells in the lungs and to a lesser extent elsewhere appears to be a frequent occurrence in both diseased and healthy subjects and the factors involved in their appearance are discussed.