The three-dimensional structures of myeloid bodies (MBs) in the retinal pigment epithelium (RPE) of the frog (Rana pipiens) were studied by transmission and scanning electron microscopy. MBs were observed to be an assemblies of numerous membranous lamellae that were formed by a flattened saccule of the paired membranes in the RPE. On thin-section, they appeared as lenticular, crescentic, circular and spherical forms and were associated with adjacent smooth endoplasmic reticulum (SER) and nuclear membranes. The lenticular MBs were observed to be in connection with the crescentic, circular and spherical forms. Under scanning electron microscopy, the lenticular and crescentic MBs showed a pile of disc-liked lamellae, while the circular and spherical MBs displayed an onion-liked appearance. It was noted that some tubular SER remained in the center of the circular MBs, while the lamellae were compactly arranged and filled the central region in the spherical MBs. All crescentic and spherical MBs were continuous with the lenticular and crescentic MBs. These results suggest that the lenticular MBs may be a precursor form of another variety of MBs which may be transformed into the crescentic, circular and spherical forms by the curving and fusion of the lamellar membranes at the MB's free ends.