Neuraminidase activity in fibroblasts obtained from a patient with combined beta-galactosidase-neuraminidase deficiency (beta-gal-/neur-) was partially restored by fusion with two ML I cell lines and an ML II cell line. As observed with neuraminidase activity, beta-galactosidase also showed complementation with an increase in activity when beta-gal-/neur- fibroblasts were fused with an ML II or a GMI gangliosidosis cell line. Both GMI gangliosidosis and sialidosis fibroblasts secreted a "corrective factor" which, when added to medium above beta-gal-/neur- fibroblasts, was pinocytosed and partially corrected its deficiencies for these two enzymes. This partial correction of beta-galactosidase and neuraminidase activities persisted for at least 72 h after removal of the "corrective factor" from the medium. A "corrective factor" with similar properties was obtained from glycoproteins isolated by chromatography of human spleen homogenates on concanavalin A-Sepharose. Treatment of beta-gal-/neur- fibroblasts with leupeptin or EP475, two inhibitors of lysosomal thiol-proteases, partially restored beta-galactosidase activity but caused no significant improvement in neuraminidase levels. The partial corrective effect of leupeptin on beta-galactosidase activity persisted for at least 2 d after removal of the drug, even in the presence of cycloheximide.