In the present study we examined the effects of menadione, a redox cycling agent, on structural changes of human osteosarcoma line 143B cells. It has been previously reported that menadione can cause necrotic or apoptotic cell death in a concentration- depending manner. In our experimental model, cells were treated with 100 microM menadione for 24 hours. Using electron microscopy technique cells carrying three kinds of morphological changes were detected: necrotic cells, apoptotic cells and those demonstrating a co-existence of apoptotic and necrotic features in one single cell.