Plasma and leucocytes from six springer spaniels with clinical signs of fucosidosis had low activities of alpha-L-fucosidase. Fucosidase activities in plasma and leucocytes from parents of springers with fucosidosis were approximately half those in non-springer dogs. Examination of pedigree information in relation to classifications based upon plasma and leucocyte assays provided convincing evidence that fucosidosis is inherited in an autosomal recessive manner and that it is possible to detect heterozygotes using plasma and leucocyte assays.