There is a deficiency of human alpha-N-acetylneuraminidase in several inherited diseases. In patients with mucolipidosis I (refs 1,2) and in adults with a variant form with out bony abnormalities and mental retardation, both also classified as sialidoses, it is the only deficient enzyme. In mucolipidosis II ('I-cell' disease) neuraminidase is one of many deficient lysosomal hydrolases and a third manifestation combines deficiency of neuraminidase and beta-galactosidase. We have investigated the genetic background of these various neuraminindase deficiencies by somatic cell hybridization and co-cultivation. The principal conclusions from work on mutant fibroblasts, reported here, are that at least three gene mutations are involved and that the combined beta-galactosidase/neuraminidase deficiency is likely to be due to defective post-translational modification of these enzymes.