AbstractAcidic α‐mannosidase deficiency has been identified in a family of Blue Persian cats. Characterization of the residual activity revealed that theKm for the substrate, 4‐methylumbelliferyl‐α‐d‐mannoside, increased approximately three‐fold with a severe deficiency inVmax (1–2%) in homogenates of liver and brain of affected cats compared with controls. The residual activity at pH 4.0 in liver homogenates from affected cats is very thermolabile at 51°C while the control activity is stable at this temperature for 1h. Subcellular fractionation of liver was performed from a control and diseased cat in order to compare the properties of the different α‐mannosidases localized in these fractions. The residual activity present in the lysosomal fraction from diseased cat liver showed altered pH optimum, two‐fold increase inKm with a severely reducedVmax and increased thermolability compared with the activity in the lysosomal fraction from control liver. The thermal inactivation pattern andKm of the residual activity in the lysosomal fraction is different from the non‐lysosomal α‐mannosidase in the liver of the affected cat. This suggests that the residual activity in the lysosomal fraction of the liver from the affected cat is not due to contamination of non‐lysosomal α‐mannosidase in this fraction. Whether this residual activity represents the properties of the mutant enzyme or yet another minor normal component of lysosomes different from the major inactive mutant or absent lysosomal enzyme remains to be elucidated.