Plasma alpha- and beta-mannosidase activities were measured in goats with beta-mannosidosis, a recently discovered, inherited deficiency of the glycohydrolase beta-D-mannosidase. Plasma beta-mannosidase activity in the affected animals was zero at 1 and 4 weeks after they were born. Enzyme values of age-matched, unaffected goats were between 66 and 222 nmol/hr/ml of plasma, alpha-Mannosidase activities were similar in both affected and unaffected animals. This investigation indicates that deficiency of plasma beta-mannosidases activity may be diagnostic of beta-mannosidosis.