The inherited deficiency of galactosylceramide beta-galactosidase (E.C. 3.2.1.46: galactocerebrosidase) activity results in globoid cell leukodystrophy in humans (Krabbe disease) and in mice (twitcher mutant). To determine whether Krabbe patients' cells complement twitcher cells to produce, in hybrid combination, greater than deficient levels of galactocerebrosidase activity, five separate crosses were made between an established twitcher mouse cell line and five cell strains from unrelated Krabbe disease patients. A total of 57 twitcher mouse/Krabbe somatic cell hybrid lines developed from all of these crosses were deficient in galactocerebrosidase activity despite the presence of human chromosomes 14 or 17, which have been previously implicated as bearing the galactocerebrosidase gene. A control cross between twitcher mouse/positive control human fibroblasts resulted in 14 of 21 independent hybrid lines that expressed higher than deficient levels of galactocerebrosidase activity. The lack of complementation between Krabbe disease patient and twitcher mutant mouse cells provides further evidence that the twitcher mouse is an authentic murine model for Krabbe disease and supports the hypothesis that the mutations in both species are within the structural gene for the galactocerebrosidase enzyme.