The loss of Rac1 activity induces a defect in neutrophil migration into the lung tissues. Mice reconstituted with WT or Rac1^−/− hematopoietic cells were exposed to fMLP in the lung. Lungs were harvested 24 hours after challenge and fixed, and lung tissue was stained with Leder stain for neutrophil esterase. (A) Representative pictures of esterase-positive (pink) cells (see arrows) infiltrated into the lung. (B) Number of esterase-positive cells per field. Mean ± SD, n = 5.