Mononuclear leukocytes isolated from the blood of rabbits, were labeled with 51Cr and returned to the animal intravenously. 51Cr-labeled monocytes disappeared from the blood with a half-life of 39.0 +/- 2.51 hours. Numerous acute inflammatory lesions were produced by the intradermal injection of Escherichia coli into the skin of the back of a rabbit. The animal was sacrificed after 1 hour, and the radioactivity in each lesion was determined. Monocyte accumulation was substantial by the time a lesion was 1 hour old. The maximum rate of accumulation occurred at 3--4 hours, and monocytes continued to enter the lesions at 25% of the maximal rate for at least 24 hours. Monocytes initially migrate into bacterial inflammatory sites simultaneously with neutrophils and histologically become the predominant cell type after 12 hours because they continue to migrate into these lesions long after neutrophils have stopped. The kinetics of monocyte migration is related to other aspects of inflammation.