If one were to design the ideal nuisance for cell cultures, the resultant might well be similar to mycoplasmas. These micro- organisms are very prevalent in nature, being found in the oral cavity, blood, the mucous membranes of the respiratory and uro- genital tract and other tissues of both man and animals. They are relatively difficult to detect microbiologically and chemically. Lacking cell walls, they do not routinely produce turbidity in cell cultures and are resistant to antibiotics that act on cell walls. Mycoplasmas grow to high titers in cell cultures. Concen- 7 8 trations of 10-10...