The genital tract of young female mice was rendered susceptible to colonisation with by pre-treating them with progesterone (usually 2·5 mg) given subcutanecously at weekly intervals for 4 weeks. Colonisation was influenced by the size of the inoculum and by the dose of progesterone; at least 2·5 x 10 organisms and at least 0·5 mg of hormone (administered on four occasions) were required. The duration of colonisation was related also to the size of the inoculum and the dose of progesterone. Similar results were obtained in TO, BALB/c and CBA strains of mice. Progesterone treatment induced the dioestrous stage of the reproductive cycle within 5 days and the cycle of the majority of untreated, mycoplasma-susceptible mice was also at this stage. However, mice, particularly of the CBA strain, were far less susceptible when not given progesterone and the mycoplasmas tended to persist for a shorter time. Mice treated with oestradiol, even in small doses, became completely refractory to infection with progesterone inhibited the growth of as did oestradiol, but vaginal washings from progesterone-treated mice were no more inhibitory than those from untreated mice. Thus, the success of progesterone in enhancing colonisation could not be attributed to a direct stimulatory effect of the hormone at the mucosal surface and we suggest that it may be due to a greater availability of progesterone-induced receptors for mycoplasmas in the dioestrous phase of the reproductive cycle than in the oestrous phase.


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