Several reports indicate that macrophage colony stimulating factor (MCSF) is one of the major factors required for osteoclast proliferation and differentiation. Paradoxically, it has also been reported that MCSF inhibits osteoclastic activity. We therefore decided to investigate in detail the effects of MCSF on resorption and osteoclast formation to try and clarify this issue. Osteoclast‐containing cultures were obtained from rabbit long bones and cultured on plastic culture dishes or devitalized bovine bone slices. MCSF (4–400 ng/ml) stimulated osteoclastic bone resorption in a time‐dependent manner and at all doses examined. After 48 h of culture in the presence of MCSF, we observed a 2‐fold increase in the total area of bone resorbed, as well as a significant increase in the area of bone resorbed per osteoclast and the number of resorption pits per osteoclast. This effect was paralleled by an increase in the number of larger osteoclasts (as determined by the number of nuclei per cell) and an increase in the size and depth of the resorption pits. Since the total number of osteoclasts remained the same, the MCSF‐induced increase in resorptive activity appeared to be related to an increase in the average size of the osteoclasts. When resorption was expressed as the amount of bone resorbed per osteoclast nucleus, larger osteoclasts resorbed more per nucleus, suggesting that large osteoclasts, as a population, are more effective resorbers than small osteoclasts. Interestingly, when osteoclasts were plated at one‐fifth the standard density, the amount of bone resorbed per osteoclast decreased considerably, indicating that resorptive activity is also affected by cell density of osteoclasts and/or of other cells present. However, at this lower density MCSF still increased osteoclast size and resorption by the same fold increase over control, suggesting that the effect of MCSF was independent of factors related to cell density.