The morphological aspects of Proteus mirabilis clearance from the urinary tract of experimentally infected mice were investigated. For this purpose, two groups of mice were compared: a group infected with the bacterium and a group that had just recovered from the infection. Differences between the two groups were found in extent of bladder and kidney inflammation, depth of infiltration into the bladder wall, and in the increment of bladder volume. There was no clear-cut difference between the two groups in number of lymphocytes and monocytes, but the number of granulocytes was significantly lower in the bladder and kidneys of the recently healed animals. Likewise, the plasma cells were fewer in healed animals than in infected ones, the difference being statistically significant for bladders and on the verge of significance for kidneys.