N-acetyl-(3H)-mannosamine [(3H)-ManNAc] was used as a precursor for the metabolic labeling of Nacetyl-(3H)-neuraminic acid [(3H)-NANA] in cultured fibroblasts of a patient with infantile sialic acid storage disease (ISSD). The metabolic fate of free and bound (3H)- NANA, isolated by high-performance liquid chromatography, was followed under pulse-chase labeling conditions. Nonsaturable accumulation of free (3H)-NANA was observed in ISSD, while the metabolic flux from (3H)- ManNAc to NANA-glycoconjugates was unaffected. Accumulated free (3H)-NANA could not effectively be chased from ISSD cells although N-acetyl-(3H)-hexosamines [(3H)-HexNAc] were appearing in the chase medium. These metabolites could arise from (3H)-NANA bound to glycoconjugates which were cleaved at normal rates in ISSD. The finding that free (3H)-NANA was markedly increased relative to its major products (3H)-HexNAc is suggestive for an impaired degradation and reutilization of (3H)-NANA due to trapping in a metabolically unaccessible pool. In titration experiments with digitonin a clear-cut increase in the latency of labeled NANA relative to a cytoplasmic marker enzyme was evident in ISSD. The release of (3H)-NANA, however, followed closely the digitonin- induced release of the lysosomal enzyme β-hexosaminidase. This is suggestive for a lysosomal location of the stored material.