SummaryGram-positive bacilli, originating from the diet, are present in the faeces of germ-free mice in this Unit. Although these organisms have never grown on culture and are assumed to be dead it was considered desirable to test this by non-cultural methods because some gut bacteria are difficult or impossible to grow in vitro by present techniques. Germ-free mice were fed a synthetic diet free from live or dead organisms for 10 days during which time the bacilli disappeared from the faeces, re-appearing when the usual diet was re-introduced. This was regarded as confirmation of the non-viability of the bacilli. The use of Wayson's stain for confirmation of non-viability was found to be inappropriate since it produced falsepositive reactions with irradiation-killed bacteria.