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Two young adult siblings were diagnosed as having a deficiency of acid beta-galactosidase activity in leukocytes and fibroblasts. The parents had enzyme levels approximately half of the normal level, consistent with this being the primary enzymatic lesion. Sialidose activities measured with natural and synthetic substrates in the patient's skin fibroblast cultures were normal. Hybridization of one of these patient's cells with cells from a patient with GM1 gangliosidosis, Type 1 did not show complementation of beta-galactosidase activity. However, when the cells from the patient were hybridized with cells from a patient with combined sialidase and beta-galactosidase deficiency, complementation was observed. These two siblings have ataxia, mild intellectual deterioration, slurred speech, mild vertebral changes and little, if any, visceromegaly. They do not have myoclonus, seizures or cherry-red spots, which are found in most patients with combined sialidase and beta-galactosidase deficiency. These patients are discussed with regard to other patients in the literature called variant or adult GM1 gangliosidosis.