Three different beta-galactosidase gene mutations--a 273Trp----Leu (mutation F) in both families, 482Arg----His (mutation G) in one family, and 509Trp----Cys (mutation H) in the other family--were identified in three patients with Morquio B disease who were from two unrelated families. Restriction-site analysis using StuI, Nsp(7524)I or RsaI confirmed these mutations. In human fibroblasts, mutation F expressed as much as 8% of the normal allele's enzyme activity, but the other mutations expressed no detectable enzyme activity. We conclude that the unique clinical manifestations are specifically associated with mutation F, a common two-base substitution, in this disease.