Isolated chicken osteoclasts in culture have been treated with 100 nM PMA for 20 minutes, and processed for the decoration of the microfilaments with fluorescent phalloidin. Results demonstrated that this phorbol ester, which activates the protein kinase C, induces the assembly of microfilaments in stress-fibers, and enlarges the microfilamentous core of podosomes. This results indicate that the protein kinase C mediates specific arrangement of microfilaments in osteoclasts. The substratum for protein kinase C-mediated phosphorylation is however still unknown.