W hen supernates from the established human breast cancer cell line MCF-7 were applied to fetal rat long bones that had been labeled with 45Ca and devitalized to remove endogenous bone cells, mineral was released from the bones. The release of bone mineral by MCF-7 supernates was associated with increased basal release of hydrolytic enzyme activity by the tumor cells. The basal release of lysosomal enzymes and collagenolytic activity by MCF-7 cells with approximately twice that of mouse 3T3 cells, which did not cause mineral release by the fetal rat bones. Release of hydrolytic enzymes and bone mineral-releasing activity was increased by colchicine and vinblastine, drugs that inhibit microtubule assembly, but not affected by lumicolchicine. Time-course experiments performed on MCF-7 cells with or without colchicine showed that release of cathepsin D and collagenolytic activity was associated more closely with release of bone mineral and degradation of bone matrix than was the release of N-acetylglucosaminidase. The release of previously incorporated [3H]proline from the bones exposed to MCF-7 cell cultures was more closely associated with release of collagenolytic activity by MCF-7 cells than with release of cathepsin D or N-acetylglucosaminidase. These data suggest that breast cancer-mediated bone resorption in vitro is positively correlated with release of hydrolytic enzymes by the tumor cells, and release of these enzymes is enhanced by disassembly of microtubules.