The conditions for assay of beta-mannosidase activity in human chorionic villi (CV) were studied using the fluorogenic substrate 4-methylumbelliferyl-beta-D-mannopyranoside. A comparison of the biochemical properties of the CV beta-mannosidase with those of the enzyme from human cultured fibroblasts showed their similarity. Like the enzyme from skin fibroblasts, the CV beta-mannosidase had rather high activity. Both enzymes had virtually the same pH optimum (4.2-4.7) and Km value. The data presented suggest that chorion biopsy specimens can be used for prenatal determination of beta-mannosidase activity at an early stage of development.