Vol. 166, No. 6, September 13, 2004. Pages 913–923.

The velocities of microspheres and neutrophils rolling on or freely moving above the flow chamber floor plotted in Figs. 2 and 4 should be reduced by a factor of 2.25. The error was caused by improper calibration of the camera magnification used for distance measurements (1 pixel = 0.278 μm instead of the originally calculated 0.625 μm). The recalculated velocities are shown below. The corrected velocities of freely flowing microspheres in the new Fig. 4 agree with theoretical predictions (Goldman, A.J., R.G. Cox, and H. Brenner. 1967. Chem. Eng. Sci. 22:653–660). The acceleration value used to set the threshold for rolling step analysis (p. 921) should also be reduced by 2.25, but this does not change the rolling step data in Figs. 5–7. We thank Dr. Daniel Hammer (University of Pennsylvania, Philadelphia, PA) for bringing this error to our attention.