Mycoplasma orale type 1 and M. hominis increased in number in the maintenance medium of cultures of L132 cells. The former mycoplasma grew slowly, but the latter grew rapidly and persisted in the cultures for more than a month. Neither mycoplasma grew in maintenance medium that had not been in contact with cells. In contrast, two T-mycoplasmas of human origin, one from the genital tract and the other from the oropharynx, grew rapidly in cultures of L132 cells and sometimes in maintenance medium from tubes without cells also, but persisted for 3–4 days only.