The behavior of Mycoplasma hominis in normal human embryonic lung fibroblast (HAIN-55) cell cultures was investigated. Multiplication patterns of cell-associated mycoplasmas and of extracellular mycoplasmas in the HAIN-55 cultures depended upon the size of the inoculum. This relationship did not vary with the number of days the cells had been cultured, nor with the number of HAIN-55 cell passages. The maximum mycoplasmal growth was obtained with inoculum sizes of 10(5) to 10(6) colony-forming units (CFU)/ml. The recovery of mycoplasmas decreased rapidly with inoculum size beyond 10(7) CFU/ml, and growth of the HAIN-55 cells was inhibited. Growth of the cells was also inhibited by the addition of the cytoplasmic fraction of Mycoplasma hominis.