Mature beta-hexosaminidase A has been found associated to the external leaflet of plasma membrane of cultured fibroblasts. The plasma membrane association of beta-hexosaminidase A has been directly determined by cell surface biotinylation followed by affinity chromatography purification of the biotinylated proteins, and by immunocytochemistry. The immunological and biochemical characterization of biotinylated beta-hexosaminidase A revealed that the plasma membrane associated enzyme is fully processed, suggesting its lysosomal origin.