The objective of this project was to develop a simple in-vitro test which could be used to obtain a phenotype which indicated an individual animal’s response to heat stress. This test could potentially overcome the current limitations to obtaining phenotypes which reflect heat stress response, which include: 1) small scale climate controlled facilities which enable heat stress to be applied, but on relatively few animals, and 2) use of indirect information from changes in performance data with season in an industry setting, which can be ambiguous (ie seasonal effects are not just about temperature) and which is not available for progeny of all sires at all times of the year. These limitations restrict the ability to select animals with improved tolerance to heat stress in commercial breeding programs. Developing an in-vitro test which overcomes these limitations could improve this aspect of breeding program development and also have application in designed experiments for quantifying the impacts of heat stress on performance and welfare.