A series of experiments was undertaken in which human embryonic kidney (HEK) fibroblasts were infected with either relatively stable or stable L-forms of Streptococcus faecalis. The infected cells were maintained by splitting over long periods (1 to 2 months) while samples were withdrawn for culture and electron microscopic studies. Relatively stable S. faecalis L-forms could be cultured from infected cells only during the first week after infection, although L-form-like material was frequently detected at later periods by electron microscopy. HEK cells continued to proliferate and showed no gross histopathology until reversion occurred. At reversion, electron microscopy revealed the presence of intracellular L-forms, transitional variants, and intra- and extracellular bacterial forms. Unlike relatively stable L-forms, stable L-forms were culturable throughout the experimental period. These experiments demonstrate the phenomena of persistence and reversion in vitro. Electron microscopy indicated that the L-forms entered the HEK cells, where they underwent morphological changes. Observations suggested that these altered L-forms (elementary bodies) persisted in the cell until reversion to the parent bacterial form was triggered. Infected cells, when treated with L-form antiserum and stained with fluorescein-conjugated goat anti-rabbit serum, were consistently positive regardless of cultural findings.