Mannosidosis of Angus cattle, previously known as pseudolipidosis, is an inherited lethal nervous disease (Whittem and Walker, 1957; Jolly, 1970, 1971). It has recently been shown that this condition is due to an inborn error of lysosomal catabolism of glycoproteins in which a deficiency of α-mannosidase results in storage of an oligosaccharide containing glucosamine and mannose (Hocking et al 1972). As such it is biologically similar to other inherited Iysosomal storage diseases of man and animals (Jolly and Blakemore, 1973). Whereas diseased calves have a near absolute deficiency of α-mannosidase, heterozygotes for this genotype have a partial deficiency of the enzyme. This observation could be exploited as a means of testing for heterozygous animals in the population at large. This paper augments earlier observations and describes some of the parameters that affect the level of α-mannosidase in normal individuals. The effects on enzyme activity of methods of collection and subsequent treatment of blood samples are also investigated.