Lines of fibroblasts have been established from normal individuals and from patients diagnosed as suffering from one of the mucopolysaccharidoses or mucopolysaccharide-storage diseases. Transformation of these lines with SV40 virus has been found to reduce their capacity to secrete sulphated mucopolysaccharides into the growth medium. No differences were detected between the individual cell types in their secretory capacity, either before or after viral transformation. A direct relationship was found to exist between the rate of acid mucopolysaccharide production and cell-doubling time. The level of sulphated mucopolysaccharide detected within the cell was also reduced for all cell types after transformation by SV40. Transformed fibroblasts from mucopolysaccharidosis patients, however, showed a relatively greater reduction in storage capacity than those derived from normal individuals.