THE use of tetrazolium salts in experimental and human pathology is valuable if the materials being examined are vital organs or tissues, for degenerating or dead tissue loses the ability to reduce tetrazolium salts1. The formation of formazan is widely used for the determination of general metabolism2. Since this enzyme reaction can only occur in the living cell, and as there are rather few methods for determining viability, the formation of formazan was investigated in ascites tumour cells during the process of degeneration in vitro.