While it is clear that the cells of many culturable pathogenic bacteria may become nonculturable but retain some cytological indices of activity and integrity, the potential for such cells to cause human disease is far from certain. Here we discuss both results and practical considerations relating to this issue. We conclude that there are no available cytological tests that satisfactorily predict whether cells have infective potential. In contrast, we recognize that nonculturable cells of pathogenic bacteria can retain substantial physiological activity, including the capacity to synthesize toxins. However, the clinical significance of these phenomena is yet to be established.