Five giant cell tumours of bone were studied to determine the degree of macrophage infiltration and whether the giant cells expressed the characteristics commonly associated with macrophages, i.e., IgGFc and C3 receptors, phagocytosis and non-specific esterase activity. Macrophages were assessed in trypsin-derived tumour cell suspensions by IgGEAC rosette formation and in frozen sections of tumour by EA adsorption. The percentage of macrophages in cell suspensions from four of the tumours ranged from 11 to 40 per cent. Strong EA adsorption occurred over 35 to 95 per cent. of the tumours' surface and significant non-specific esterase positivity was observed in the tumour sections. The giant cells were receptor negative and non-phagocytic, but a low percentage of them expressed esterase activity. The results strongly suggest that despite the fact that large numbers of macrophages were present in the tumours, the giant cells were derived from cells other than macrophages.