Human GM1-gangliosidosis is caused by a genetic deficiency of lysosomal acid beta-galactosidase (beta-gal). The disease manifests itself either as an infantile, juvenile or adult form and is primarily a neurological disorder with progressive brain dysfunction. A mouse model lacking a functional beta-gal gene has been generated by homologous recombination and embryonic stem cell technology. Tissues from affected mice are devoid of beta-gal mRNA and totally deficient in GM1-ganglioside-hydrolyzing capacity. Storage material was already conspicuous in the brain at 3 weeks. By 5 weeks, extensive storage of periodic acid Schiff-positive material was observed in neurons throughout the brain and spinal cord. Consistent with the neuropathology, abnormal accumulation of GM1-ganglioside in the brain progressed from twice to almost five times the normal amount during the period from 3 weeks to 3.5 months. Despite the accumulation of brain GM1-ganglioside at the level equal to or exceeding that seen in gravely ill human patients, these mice show no overt clinical phenotype up to 4-5 months. However, tremor, ataxia and abnormal gait become apparent in older mice. Thus, the beta-gal-deficient mice appear to mimic closely the pathological, biochemical and clinical abnormalities of the human disease.