The sterilising efficiency of 0.2-mu m rated membranes is validated via a retention test using the small bacterium, Pseudomonas diminuta. Whilst the demand for such validation has increased, the tests are regarded as lengthy and are not always routinely carried out, Unfortunately, the results from integrity tests alone are not accurate enough to ascertain the sterilising ability of membranes. This review describes the current situation for the retention testing of 0.2-mu m rated membranes after first giving an outline of the process of sterilising filtration and of the retention test organism. Suggestions for the development of the standard retention test procedure are then given, namely the development of a rapid method of determining permeate sterility and also of a means of retention testing membranes during cross-flow filtration.