Toothless is an osteopetrotic mutation in the rat characterized by reduced bone resorption, few osteoclasts and failure to be cured by bone marrow transplants from normal littermates. The latter could be due to the production of bone that cannot recruit or activate normal osteoclasts or their precursors. We tested this hypothesis using a metatarsal organ culture system in which metatarsals are cultured with various tissues which act as sources of osteoclast precursors. Mutant metatarsals cultured alone or with mutant tissue had numerous large mononuclear cells, few osteoclasts, no marrow spaces and no evidence of bone resorption. Culture of mutant metatarsals with normal liver or spleen caused no changes in osteoclast number or bone resorption. Cultures of normal metatarsals had large osteoclasts, bone resorption and marrow spaces. Histochemical analyses demonstrated that strong tartrate-resistant acid phosphatase (TRAP) staining was present in all osteoclasts and in some mononuclear cells in normal metatarsals. Mutant metatarsals before or after culture contained no TRAP-positive osteoclasts and an occasional, weakly staining mononuclear cell only when co-cultured with normal spleen which contained TRAP-positive mononuclear cells. We interpret these data to mean that the mutant skeletal environment is unable to support the development and differentiation of normal osteoclasts. These observations are consistent with the failure of toothless rats to be cured by bone marrow transplants from normal littermates.