The immunocytochemical localization was investigated of a major lysosomal membrane sialoglycoprotein with a molecular mass of 107 kDa, which was designated as LGP107. The study utilized rat osteoclasts with different bone resorbing activity and osteoclast precursors at various stages of differentiation and maturation together with monospecific antibodies to this protein. Despite its localization primarily in lysosomes and endosomes in the other cell types examined, LGP107 was exclusively confined to the apical plasma membrane at the ruffled border of the active osteoclast, where the osteoclast is in contact with the bone surface. The protein was also concentrated in a number of endocytic vacuoles in the vicinity of the ruffled border membrane. However the labeling was not found in the basolateral membranes of the active osteoclast. The ruffled border membrane detached from the bone surface showed a marked decrease in the extent of the immunolabeling. The post- and/or resting osteoclasts, which were located away from the bone surface, were totally devoid of the membraneous localization of LGP107. No definite immunolabeling was found in the immature preosteoclasts. These results indicate that the protein is largely synthesized in the active osteoclast and rapidly translocated to the ruffled border membrane by vectorial vesicle transport. LGP107 is suggested to contribute to the formation and maintenance of the specialized acidic environment for bone resorption.