Characterisation of non-toxigenic Clostridium spp. strains, to use as surrogates for non-proteolytic Clostridium botulinum in chilled food challenge testing. Under many of the conditions studied, a two-strain cocktail of non-toxigenic Clostridium spp. was found to be suitable as a surrogate for non-proteolytic Clostridium botulinum, and has the potential for use in chilled food challenge tests measuring growth. Non-toxigenic surrogates could also be used in thermal process screening studies.