We sought to ascertain whether an in vitro assay could be as reliable as an in vivo assay in determining the viability of human skin stored at 4° C. Allografts from six human donors were stored in RPMI 1640 tissue culture medium at 4° C. At fixed intervals during the storage period, all skin specimens were tested concurrently by two different viability assays: (1) transplantation onto surgically created defects on nude mice, and (2) intracellular enzyme activity with use of a 4-hour semiquantitative micromethod system activity (API ZYM; Biomerieux Vitek Inc., Hazelwood, Mo.). Human graft survival on the nude mice was 100% for the first 15 days of storage, and then declined to 50% on storage day 30. The API ZYM assay showed a comparable progressive decrease in enzyme activity over skin storage time. The API ZYM assay is a simple, rapid system that produces reproducible results and is cost-effective when compared to the biologic model.