Summary Electronmicrographs of sections show that the L-phases of Bacillus licheni-formis var. endoparasiticus (Benedek) simulate the various types of microorganism described by previous workers as associated with ostensibly non-infective conditions, notably cancer and arthritis; e.g., mycoplasmas, myco-bacteria, corynebacteria and actinomycetes. The stages of the L-cycle, from spheroplasts through smaller and larger ‘diphtheroid’ bacteria to the fully reverted sporogenous bacillus, differ from one another mainly in their degree of reconstitution of the cell envelopes. Occurrence in ‘diphtheroids’ of non-resistant, cell-wall-deficient spore-like bodies, confirms their relationship with the true sporing bacillus stage. Large, swollen forms appear to be mother cells of minute stages.