The effects of lanthanum （Ⅲ） on the bone resorbing activity of rabbit mature osteoclasts （OCs） in the presence of osteoblasts （OBs） were studied in vitro by measuring the number and area of absorption pits. La（Ⅲ） at concentrations ranging from 1.00×10-5 to 1.00×10-8 mol·L-1 show no effect on mature OC number （P>0.05）. In the OC-OB co-culture systems without La（Ⅲ）, osteoblasts alone did not influence the pit number and area whether the two kinds of cells were in contact or not （P>0.05）. Under the OC-OB not-in-contact condition, the effect of La（Ⅲ） on the bone-resorbing activity of OCs was similar to that of La（Ⅲ） in the absence of OBs （P>0.05）. However, while OCs were in direct contact with OBs, the inhibitory effects of La（Ⅲ） on OCs′ bone-resorbing activity decreased at the concentrations of 1.00×10-5, 1.00×10-6 and 1.00×10-7 (mol·L-1), and the promotion effects increased at 1.00×10-8 (mol·L-1) （P<0.05）. The results suggest that direct cell-cell contact between OC and OB be essential for OBs to play their role in regulating the response of OCs to La（Ⅲ）.
