This study examined the relationship between the number of nuclei in an osteoclast and its resorptive efficiency, as demonstrated by the size of the pit it can make in a mineralized tissue in 24 h in vitro. Osteoclasts released mechanically from prehatch chick long bones were cultured on dentine slices or on plastic dishes for periods of 6 or 24 h. The frequency distribution of the multinucleate tartrate-resistant acid phosphatase (TRAP)-positive cells with different numbers of nuclei was determined: the mean number of nuclei per cell was 6.92, with a mode of 4. 47% had 5 or fewer nuclei and only 11% more than 10 nuclei. The pits associated with 292 osteoclasts with known numbers of nuclei were measured using a confocal laser light microscope (Lasertec) and dedicated image analysis system, and depths, plan areas and volumes determined. There was a positive correlation between the number of nuclei per osteoclast and the volume of the pit made, but a trend for the volume resorbed per nucleus to decrease with increase in the number of nuclei per osteoclast.