Residual acidic alpha-mannosidase, varying in amount up to approx. 15% of normal values, can be measured in various organs of a calf with mannosidosis. The highest specific activity and relative proportion of residual activity were found in the liver. Chromatography on DEAE-cellulose showed that the residual activity was associated with two components, which were eluted at comparable positions with those found in normal tissues. The residual activity had a lower thermal stability and a higher K(m) value for a synthetic substrate than did the normal enzyme. No differences in molecular weight or electrophoretic mobility between normal acidic alpha-mannosidase and the residual activity were observed by gel filtration and electrophoresis on cellulose acetate respectively. The isoelectric focusing profiles for the alpha-mannosidase in the normal and pathological livers were very similar. It is suggested that a mutant enzyme, resulting from a mutation in a structural gene, accounts for the residual acidic alpha-mannosidase in mannosidosis. The mutant enzyme, which cross-reacts with antiserum raised against normal bovine acidic alpha-mannosidase, is present at a decreased concentration compared with the normal enzyme. There is a correlation between the concentrations of residual activity and cross-reacting material in mannosidosis. alpha-Mannosidase with a pH optimum of 5.75 and which is activated by Zn(2+) was also detected in the liver of the calf with mannosidosis. However, it is probably not a product of the defective gene because addition of Zn(2+) indicated that it was also present in normal tissues.