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 colony forming units per ml of supernatant medium are representative. Additionally, more numbers are attached to cell membranes. Further, mycoplasmas have been shown to mimic in vitro effects of viruses and toxic chemicals. In various cell culture systems, mycoplasmas have been known to cause cell death, decrease or increase cell growth, affect virus tite~s, induce interferon, cause chromosome damage, induce transformation, cyto­ pathic effects, alter phenotypic expression, and significantly alter metabolic pathways and products of cells. The presence of such high concentrations of mycoplasmas in cell cultures constitutes a true in vitro infection. Such infected cell cultures, with a 8 9 total of 10 _10 or more actively metabolizing mycoplasmas, have no place in controlled, standardized cell culture procedures. Numerous reports have been published on mycoplasma assay pro­ cedures, effects of infection, and preventive and elimination measures.