Abstract In human liver it is clearly evident that the levels of histidase and urocanase activities are considerably lower than those encountered in mouse liver. Both in the mouse and in the man urocanase presents levels of activity much higher than histidase. The apparent K m of histidine for the mouse histidase was 2.71 · 10 −4 M, whereas the K m for the human histidase was 4.83 · 10 −4 M; the apparent K m value of urocanase was 2.18· 10 −6 M for man, and 3.19sd 10 −3 M for the mouse. In mouse hepatitis there is a decrease of both enzyme activities in the liver and at the same time their passage in blood is observed. In human viral hepatitis the levels of both enzymes in blood plasma increase in an inconstant manner and no relationship can be observed with the extent of the hepatocellular damage.