The immunostimulatory drug tilorone (CAS 27591-97-5) induces generalized lysosomal storage of sulfated glycosaminoglycans, i.e., mucopolysaccharidosis (MPS) in rats. Since inherited MPS of man is often associated with skeletal lesions the present study was performed to investigate whether the tilorone-induced MPS is also accompanied by bone alterations. The answer is yes. Rats (initial age 4 weeks) were orally treated with tilorone (60-80 mg/kg; 6-25 weeks). The proximal tibial metaphysis was examined by radiography and by light and electron microscopy. The tibiae of drug-treated rats remained smaller than those of pair-fed control rats. The primary bone trabecules in drug-treated rats became increasingly shorter, and the cortical bone became thinner. The general organization of the cartilaginous growth plate and the zones of vascular invasion and primary trabecule formation appeared similar as in control rats. Increased numbers of active osteoclasts were found at the free ends of the primary trabecules. Ultrastructurally, MPS-like alterations were observed in all types of bone cells. The osteopenia was partly reversible during recovery periods of more than 6 months after discontinuance of drug treatment. At present, the causal relationship between drug-induced MPS and osteopenia, as well as the mechanisms responsible for osteopenia are unknown. It is suggested that the principal factors are stimulation of osteoclasts and retardation, though not profound distortion, of the events involved in enchondral ossification.