Osteopetrosis is a metabolic bone disease characterized by reduced bone resorption. From experimental studies of various osteopetrotic mutations has emerged the hypothesis that each is unique with respect to mechanisms whereby osteoclast development and/or function are reduced. The osteopetrotic (op) mutation in the rat was discovered in Fatty/ORL stock over a decade ago. The paucity of data about osteoclast biology in this mutation prompted this study of cytological, cytochemical, and ultrastructural features of osteoclasts. In op rats, osteoclasts are significantly reduced in number, but are larger and more vacuolated than in normal littermates. Mutant osteoclasts can form ruffled borders and clear zones, but their ability to fragment and excavate bone surfaces is greatly impaired. Cytoplasmic vacuoles in op osteoclasts are randomly distributed and greatly enlarged, and they stain weakly for two cytochemical characteristics of osteoclasts, tartrate-resistant acid phosphatase and acid ATPase. These findings suggest that an abnormality in the lysosomal/vacuolar system, an important component of the resorptive mechanism, may be involved in the interception of osteoclast function in this mutation.