A combination of microscopical, cytochemical, and biochemical techniques have been employed to study the changes occurring during the first seven days of blastema formation and regeneration after decapitation in adult Polycelis tenuis worms. Fine structural data reveal evidence of cell fragmentation, selective cell deletion, and phagocytosis at and below the wound surface. Initially, (0-12 h regeneration) cell debris is phagocytosed by intact parenchymal and gastrodermal cells near the cut surface which is later sealed (24 h) by a stretching of marginal epidermal cells. Wound sealing is followed by a migration of newly differentiated rhabdite cells into the epithelium. Morphological evidence of a selective cell autolysis precedes evidence of an accumulation of lipid and glycogen reserves in the parenchymal and gastrodermal cells and the later (48 h regeneration time) aggregation of undifferentiated mitotically active neoblasts beneath the wound. Biochemical data reveal an early period of high acid phosphatase (p-nitrophenyl phosphatase and sodium-beta-glycerophosphatase) activity 3-12 h after injury, followed by a further intense period of activity at 44-48 h after decapitation. The coincident cytochemical data show an increased level of acid phosphatase activity associated with cell lysis and death in the wound and blastema zone and also with the digestion of phagocytosed cell debris.