The activity of beta-hexosaminidase, determined with 4-methylumbelliferyl-beta-N-acetylglucopyranoside substrate, and of beta-D-mannosidase was significantly higher in the serum of patients with carbohydrate-deficient glycoprotein (CDG) syndrome type IA (phosphomannomutase deficiency) than in controls. No significant differences were observed in the activity of beta-hexosaminidase, determined using 4-methylumbelliferyl-beta-N-acetylglucopyranoside-6-sulphate as substrate, and the activity of alpha-D-mannosidase. Using DEAE-cellulose chromatography, a greater amount of hexosaminidase B than hexosaminidase A was detected in CDG serum. In CDG serum, hexosaminidase A was eluted in a more basic position in the salt gradient. An isoenzyme of alpha-D-mannosidase and beta-D-mannosidase was identified in control and CDG sera. alpha-D-Mannosidase isoenzyme was eluted in a slightly more basic position in CDG serum than in control serum, whereas beta-D-mannosidase isoenzyme was eluted in the same position.