Mice of strain TO, in groups of ten, were inoculated intravaginally with Mycoplasma pulmonis organisms. Seven mice became infected after inoculation of organisms with strong haemadsorptive capacity, four after inoculation of organisms with diminished adsorptive capacity following ten passes in medium, and none after inoculation of apparently non-adsorbing organisms which had been passed 50 times. There appeared to be a correlation, therefore, between the ability to infect and the cytadsorptive capacity of the organisms. There was only a minimal vaginal polymorphonuclear leucocyte (PMNL) response in the infected mice and most of them had ceased to be infected by 35–42 days. In contrast, mice treated with progesterone had enhanced infections; all those given strongly haemadsorbing organisms, and organisms passed ten times, became infected and remained so for at least 42 days. Furthermore, at least ten fold more organisms were recovered from progesterone-treated than from untreated mice, and the PMNL response was much greater. Most of the progesterone-treated mice given organisms passed 50 times did not become infected, but some did, and the organisms recovered from them were fully cytadsorptive. It is postulated that a few cytadsorbing organisms in this inoculum were induced to infect under the enhancing effect of progesterone.