According to the present invention, there is provided a method of analysing a sample of free cells in vitro comprising the steps of: applying a known or identifiable quantity of stress to the sample; measuring the sample before and after the application of stress to provide at least one reading from which quantitative information relating to the number of cell fragments in the sample caused by the applied stress can be determined; and, relating this reading to the quantity of stress to provide an indication of cell friability.