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.