formation of insoluble maroon deposits at sites of activity. Tartaric acid was used in order to demonstrate the presence of tartrate-resistant acid phosphatase. Hence, cells containing tartrate-sensitive acid phosphatase are devoid of activity, and only the cells containing tartrate acid-resistant phosphatase show maroon dye deposits at the sites of activity. Staining confirmed that the presence of TRAP-positive multinucleated osteoclastlike cells was limited to those wells dosed with exogenous M-CSF and RANKL, under the cell culture conditions described (Fig. 5) .