The following 3 parameters were studied in Morris hepatomas of different growth rates: the specific activity of GDP-fucose:glycoprotein fucosyltransferase and CMP-N-acetylneuraminic acid:glycoprotein sialyltransferase; the content of GDP-fucose and CMP-N-acetylneuraminic acid; and the activity of .alpha.-L-fucosidase and neuraminidase. Fucosyltransferase activities were significantly elevated in all hepatomas investigated. Especially high levels of enzyme were measured in the rapidly growing tumors 7777, 66 and 3924A. The increase varied between 2- and 3-fold when compared with the corresponding host liver. The activity of sialyltransferase was decreased in all hepatoma lines with a rapid or intermediate growth rate. In the fast growing tumor 9618A2, the activity was reduced to 8%. GDP-fucose and CMP-N-acetylneuraminic acid were determined by the isotope dilution technique. In normal rat liver from Buffalo or Acl rats, the concentration of GDP-fucose was 6.5 .+-. 0.9 and 9.5 .+-. 1.1 nmol/g, wet weight, respectively. In the fast growing hepatomas 3924A and 9121, levels up to 21.5 nmol/g, wet weight, were found. The content of CMP-N-acetylneuraminic acid in hepatomas was influenced to a lesser extent by the degree of differentiation of the tumor. In the most rapidly growing tumor, 9618A2, a level of .alpha.-L-fucosidase 7 times higher than in host liver was determined. A correlation existed between the age of the hepatoma and enzyme activity. Within the 2nd wk after inoculation, fucosidase activity increased from 130 to 343 nmol/h per mg of protein. Neuraminidase was measured in a new linked assay system. The activity of this enzyme was lowered by 50% or was at least unchanged when compared to the activity in host liver. The results indicate that specific alterations of fucose metabolism are a characteristic feature of Morris hepatomas.