A beta-glucuronidase deficiency found in serum, leukocytes and fibroblasts and an increased [35S]sulphate incorporation in fibroblasts led us to diagnose two cases of type VII mucopolysaccharidosis in one family. In spite of the wide distribution of activities in serum from controls, decreased beta-glucuronidase activity allowed us to demonstrate the heterozygous status of the parents and two other children. Following these studies, and antenatal diagnosis was performed when the mother was pregnant again; amniotic fluid and cultured amniotic cells were used for enzyme activity determination. A heterozygous fetus was suspected and confirmed after birth. The reliability of various biological materials for enzymatic diagnosis and existence of genetic variants in the normal population are discussed.