Cathepsin B-encoding cDNA (CTSB) clones have been isolated from a lambda gt10 library of a murine osteosarcoma by differential screening during a search for genes which are typically expressed during osteogenic differentiation in mouse mandibular condyles in vitro. Sequencing of the CTSB 3' end revealed that the isolated sequence contained an 825-bp 3'-noncoding region, the polyadenylation signal and the poly(A) tail. The enhanced CTSB expression during the early stages of the enchondral ossification-like process in mandibular condyles in vitro suggests that CTSB participates in the degradation of cartilage matrix prior to the synthesis of bone matrix proteins.