Glycogen-storage disease type II (GSDII) is caused by the deficiency of lysosomal alpha-glucosidase (acid maltase). This paper reports on the analysis of the mutant alleles in an American black patient with an adult form of GSDII (GM1935). The lysosomal alpha-glucosidase precursor of this patient has abnormal molecular features: (i) the molecular mass is decreased, (ii) the phosphorylation is deficient and (iii) the proteolytic processing is impaired. Sequence analysis revealed four mutations leading to amino acid alterations: Asp-645-->Glu, Val-816-->Ile, Arg-854-->Stop and Thr-927-->Ile. By using allele-specific oligonucleotide hybridization on PCR-amplified cDNA we have demonstrated that the Arg-854-->Stop mutation is located in one allele that is not expressed, and that the other allele contains the remaining three mutations. Each of the mutations was introduced in wild-type cDNA and expressed in COS cells to analyse the effect on biosynthesis, transport and phosphorylation of lysosomal alpha-glucosidase. The Val-816-->Ile substitution appeared to have no significant effect in contrast with results [Martiniuk, Mehler, Bodkin, Tzall, Hirshhorn, Zhong and Hirschhorn (1991) DNA Cell Biol. 10, 681-687] and was therefore defined as a polymorphism. The Thr-927-->Ile substitution deleting one of the seven glycosylation sites was found to be responsible for the decrease in molecular-mass, but not for the deficient proteolytic processing and phosphorylation. It did not cause the enzyme deficiency either. The third mutation leading to the Asp-645-->Glu substitution was proven to account in full for the observed defects in transport, phosphorylation and proteolytic processing of the newly synthesized alpha-glucosidase precursor of the patient.