The inflammatory response to Escherichia coli was quantitated in the skin of rabbits. The lesions were studied up to 24 hours. The vascular permeability increase, measured with 125I-albumin, reached a peak 2 hours after injection of bacteria and subsided after 4 hours. Hyperemia measured with 57Co- or 113Sn-labeled microspheres, reached its maximum value around 3 hours, and diminished after 5 hours. In the 24-hour-old lesions both vascular permeability and hyperemia remained at values twice as high as those of control skin sites. The rate of leukocyte accumulation was measured using 51Cr-labeled autologous leukocytes. It reached peak values by 2 hours. Microscopic examination showed that the number of neutrophils remained almost unchanged between 4 and 24 hours after the injection of bacteria. 59Fe-labeled erythrocytes were used to quantitate hemorrhage. Hemorrhage was measurable by 2 hours and plateaued after 4 or 6 hours. It remained at the plateau levels for the 24 hours of the experiment.