1. Deoxyuridine triphosphatase (dUTPase) in human hepatoma was investigated. The apparent activity in a gram weight tissue was about 6 times that of the activity in rat livers after partial hepatectomy. 2. Most catalytic properties of the hepatoma dUTPase were similar to the enzymes from other sources. 3. The hepatoma dUTPase reacted with the antibodies against the rat spleen dUTPase, although the molecular size of the enzyme (46-48 kDa) was different to the rat enzyme. 4. The immunochemical studies indicated that the enzyme was composed of two identical subunits, whose molecular mass was about 22 kDa.