Studies on Mycoplasma pulmonis and Mycoplasma neurolyticum mitogenesis demonstrated that macrophages are not essential for the interactions of M. neurolyticum with B lymphocytes, whereas M. pulmonis that stimulates B and T lymphocytes depends on macrophages to fully exert its mitogenic effects. Membranes of both species carry the mitogenic as well as pathogenic potentials. However, M. neurolyticum major mitogens are biochemically distinct from those of M. pulmonis. Furthermore, M. pulmonis mitogenesis is directly correlated to the mycoplasmal-induced pathogenic effects, while no such correlation could be established with M. neurolyticum. It was, therefore, concluded that M. pulmonis and M. neurolyticum differ in the biochemical and biological natures of their mitogens.