When the cell wall of Bacillus subtilis is removed by lysozyme and the resultant protoplasts are plated on hypertonic soft agar medium, each protoplast forms an L colony. L bodies from such L colonies again plate as L-colony-forming units (CFU). However, if protoplasts or L bodies are "conditioned" by 1 h of incubation in 0.4% casein hydrolysate medium and then incubated in 25% gelatin medium for 1 h, 60 to 100% of the formerly naked cells give rise to bacillary colonies. The present experiments largely explain the mechanism responsible for the "heritable" persistence of the wall-less state in B. subtilis. It is shown that protoplasts produce a reversion inhibitory factor (RIF) which blocks reversion when the cell concentration exceeds 5 x 105 CFU/ml. This inhibitor is nondialyzable and sensitive to trypsin, heat, and detergent. Efficient reversion at 2 x 107 CFU/ml is obtained if the protoplasts are treated with trypsin after conditioning and chloramphenicol is incorporated into the gelatin reversion medium. In the presence of 500 ,ug of trypsin per ml, the requirement for gelatin is sharply reduced, and reversion occurs rapidly in liquid medium containing only 10% gelatin. Trypsin also stimulates reversion in L colonies growing on soft agar. Latent RIF is activated by ,3-mercaptoethanol. This reagent blocks reversion of protoplast suspensions at densities of 5 x 105 CFU/ ml. Comparison of the autolytic behavior ofB. subtilis and of the RIF revealed that several of the properties ofthe two activities coincide: both are inhibited by high concentrations of gelatin, both are activated by (3-mercaptoethanol, and both have high affinity for cell wall. Going on the assumption that REF is autolysin, models for protoplast reversion and L-form stability are proposed. A role of teichoic acid in reversion is suggested by the finding that mutants with altered teichoic acid show altered reversion behavior.When the cell wall is removed from Bacillus subtilis with lysozyme and the resultant protoplasts are plated on hypertonic soft-agar medium, each protoplast gives rise to an L colony consisting of wall-less, irregularly shaped L bodies (16,24). L bodies from suspended L colonies again give rise to L colonies in further platings (16). Thus, a brief transient enzyme treatment produces an alteration in each member of the treated population-namely, walllessness-which is then passed on "heritably" through an indefinite number of cell generations. We have called these naked offspring "mass conversion stable L forms" (13) to underline the distinction between these L forms and certain kinds of gene mutants blocked in peptidoglycan or peptidoglycan precursor biosynthesis, which are also sometimes called stable L forms (see, for instance, 28). We are quite certain that the persistent change induced in mass Present address: