Organ cultures of newborn mouse calvaria were used to test the hypothesis that tartrate-resistant acid phosphatase might serve as a biochemical marker for osteoclast function. When bone resorption was stimulatedin vitro with either parathyroid hormone or 1,25(OH)₂D₃, there was a significant increase in both tartrate-resistant and tartrate-sensitive acid phosphatase activity in the medium relative to cultured controls. Tartrate-resistant activity was localized histochemically primarily over the osteoclast and appeared as three distinct activity bands when electrophoresed on polyacrylamide gels. The tartrate-sensitive activity was found primarily associated with bone cells other than the osteoclast using histochemical techniques, and was resolved into five bands on polyacrylamide gels. The results obtained from biochemical assays, histochemical observations, and polyacrylamide gel electrophoresis suggest that bone resorptionin vitro results in the release of tartrate-resistant acid phosphatase from osteoclasts and tartrate-sensitive acid phosphatase from other bone cells as well as osteoclasts. Tartrate-resistant acid phosphatases of bone may be suitable biochemical probes for osteoclast function, but it will be necessary to achieve further purification in order to develop analytical methods with sufficient sensitivity and specificity (e.g., immunochemical) to ensure precise localization and quantitation.