Inapparent chlamydial and rickettsial infections are an important source of the dissemination of the parasites and may cause explosive outbreaks of severe diseases in man and animal. However, it is engimatic how these obligate intracellular microbes induce latency and why or how inapparent infections convert into active disease. Currently, microbiologists believe that chlamydiae and rickettsiae are gram-negative bacteria unique in their intracellular habitat. This review presents evidence that these organisms have another peculiarity; namely, defective cell walls present throughout much of their life cycle. In this survey, the properties of the small and large forms of chlamydiae and rickettsiae are reexamined with regard to the current knowledge of cell wall defective variants of free-living bacteria. Data are presented in support of the concept that chlamydiae and rickettsiae are cell wall defective microbes whose small 'bacterial' forms have lost the ability to reproduce as bacteria during evolution; the large forms including ultrafilterable phases of the life cycle of these parasites are responsible for inapparent infections of healthy carriers, whereas conversion into small 'bacterial forms may cause, under appropriate conditions manifest diseases. This concept may provide challenging and profitable directions in the search to explain puzzling phenomena associated with chlamydiae and rickettsiae.