SUMMARYThe FH strain ofMycoplasma pneumoniaewas studied in experimentally infected cultures of lung diploid fibroblasts. The multiplicity of infection was found to have a profound influence on the distribution of the cell-associated and extra-cellular mycoplasmas. At a high multiplicity there was very little difference between the ratio of cell-associated and extracellular mycoplasmas. In contrast, with a low multiplicity the cell-associated mycoplasmas predominated and with further reductions of the inoculum only the cell-associated mycoplasmas could be detected. The significance of these findings is discussed. It is suggested that caution has to be exercised in the determination of the potency of antibiotics against mycoplasma by methods employing acellular media alone, and it is desirable to utilize also appropriate cell culture models. A possible approach to designing such models is provided by this study.We express our appreciation to the Tissue Culture Laboratory, Pfizer Limited, Sandwich, for kindly supplying us with cell cultures. We furthermore thank the Board of Directors, Pfizer Limited, for permission to publish this study. The helpful discussions with our colleagues Drs D. Jackson and G. M. Williamson in preparing the manuscript are acknowledged with thanks.