Effects of age, sex, and reproductive status on caprine plasma alpha- and beta-mannosidase activities were studied. The potential use of these plasma assays was evaluated for heterozygote detection of caprine beta-mannosidosis in populations of control goats and a breeding herd composed of known and putative heterozygotes for beta-mannosidosis. In the control population, the mean plasma alpha- and beta-mannosidase activities decreased in both sexes with maturity. Male goats generally had higher plasma mannosidase activity than did females, and plasma beta-mannosidase activity was affected by the reproductive status of the goats examined. Although putative carriers in the beta-mannosidosis breeding herd could not be differentiated from age- and sex-matched controls, known carriers had plasma beta-mannosidase values intermediate between those of control and affected goats. The lack of any correlation between alpha- and beta-mannosidase activities excluded alpha-mannosidase as a reference enzyme for carrier detection, but there are still differences between known carriers and the control population with respect to the alpha- and beta-mannosidase ratio.