The present work has been undertaken to study by scanning electron microscopy (SEM) the cell surface changes occurring in granulocytes and macrophages during the process of maturation in soft agar cultures. All stages of maturation of these cells can be seen in a single colony, with immature cells at the center and mature cells toward the periphery. Special emphasis was made, therefore, to maintain the integrity of the colonies during preparation of the specimens for the SEM. Colonies of granulocytes appeared as tight aggregates of cells. The surfaces of cells in such colonies in 3 to 4-day-old cultures contained many microvilli. Cells in colonies of 5 to 7-day-old cultures contained fewer microvilli on their surfaces. Colonies of macrophages appeared as loose aggregates of cells. The surfaces of cells in such colonies in 3 to 4-day-old cultures was slightly ruffled, while the surfaces of cells in 5 to 7-day-old colonies had well developed ruffles. Granulocyte and macrophage colonies showed the same internal organization of the colony: In granulocytes, cells with many microvilli were located in the center of the colony and cells with less microvilli were at the periphery of the colonies. In macrophages, cells slightly ruffled were located in the center of the colony, while cells highly ruffled were at the periphery of the colonies. These observations have been confirmed by transmission electron and light microscopy preparations.