We report the induction of apoptosis in a human neuroblastoma cell line SK-N-BE(2) by cisplatin or retinoic acid, and its relation to cell cycle. Apoptosis was monitored by counting apoptotic bodies and evaluating the activity of 'tissue' transglutaminase (EC 220.127.116.11), one of the genes specifically expressed in apoptotic cells. Data indicate that both agents enhance apoptosis, even though cells arrest at different cell cycle phases. In fact, retinoic acid causes accumulation in G1, whilst cisplatin induces accumulation of cells in the G2/M phase. This evidence suggests the presence of multiple start points for the apoptotic death programme within the cell cycle of human neuroblastoma cells.