Deletion of Rac1/Rac2, but not Rac1 alone, in T cells affects the development and proliferation/survival/adhesion/migration of thymocyte T cells. Lck-Cre–targeted mice were injected intraperitoneally with BrdU (100 μg/g body weight) 12 hours prior to harvesting thymus. Isolated thymocytes were quantified for the absolute number (A) and stained with anti-CD4, -CD8, -CD44, -CD25, -CD69, -HSA, -CD62L, and/or -BrdU antibodies or annexin V. The stained cells were subjected to flow cytometry analysis of the CD4/CD8 subpopulations (B), the expression level of HSA/CD62L in CD4⁺ or CD8⁺ SP thymocytes (C), the CD44/CD25 expression in CD4⁻CD8⁻ DN subpopulations (D), the CD69 level in CD4⁺CD8⁺ DP thymocytes (E), the proliferating cells (F), and the apoptotic cells (G) in various T-cell subpopulations. The adhesion to fibronectin and migration to SDF-1α of total thymocytes (H) or migration of CD4⁺ or CD8⁺ SP thymocyte to MIP-3β (I) was also determined. WT, n = 10; Rac1^−/−, n = 6; and Rac1^−/−Rac2^−/−, n = 6. *P < .05; **P < .01. Error bars represent SD.