To detect whether mycoplasmas produce p53 inhibitory factors, we incubated mouse Balb 3T3 cells stably transfected with the p53-responsive β-galactosidase (β-gal) reporter gene (Komarova et al., 1997) with filtrates of cell-free broths conditioned with different in vitro grown mycoplasma species (M. fermentans, M. arginini, M. hominis, M. arthritidis)