The twy (tiptoe-walking-Yoshimura) mouse is a mutant having a systemic hyperostotic disposition. Osseous overgrowth occurs not only in the spine but also in other skeletal tissues including the calvarium. To clarify the pathogenic mechanism of hyperostotic lesions, bone cells were cultured from twy and normal mouse calvariae to analyze the growth and extracellular matrix production in both genotypes. The cultures of osteoblast-enriched cells were established by explant procedure. The cells of the twy genotype showed accelerated growth and elevated levels of collagen synthetic activity compared to normal cells. However, no significant difference was noted in noncollagenous protein synthesis between the two genotypes. Furthermore, the mutant cells maintained these abnormal functions during several cell passagesin vitro. These results indicated that an unusual phenotype of twy bone cells distinguished by accelerated growth and selective stimulation of collagen production may play a major role in the development of hyperostotic lesions in twy micein vivo.