Frozen stored murine hybridoma cells can be used to determine the virulence of Listeria monocytogenes.

Murine hybridoma cells, designated Ped-2E9, when stored up to 60 days at -196 degrees C or up to 48 days at -80 degrees C, gave results equivalent to those for freshly grown murine hybridoma cells in an in vitro pathogenicity assay of Listeria species. Thus, laboratories do not need to have their own tissue culture facilities to maintain the hybridoma cells for the assay described.