Tyrosine kinase p56lck plays a pivotal role in beta-selection from CD4-8- (DN) to CD4+8+ (DP) developing pathway, but it is unclear how CD45 transmembrane tyrosinephosphatase is involved in this process although CD45 activates p56lck by dephosphorylating its tyrosine-505. To analyze this issue, we produced double mutant mice of T-cell receptor transgenic mice (TCR-Tg) or RAG-2 knock out mice backcrossed with either p56lck or CD45 knock out mice. In TCR-Tg, CD25+DN thymocytes almost disappeared and CD25-44-DN cells of further developing stage increased, implying that all DN thymocytes can undergo beta-selection due to the expression of functionally rearranged TCR-beta on CD25+ DN thymocytes. However, CD25+ thymocytes increased in DN stage when TCR-Tg were backcrossed with p56lck deficient mice but not with CD45 deficient mice. Similarly, DP thymocyte induction with CD25+ cell reduction in RAG-2 knock out mice by injection of anti-CD3 mAb was inhibited in p56lck deficient but not in CD45 deficient mice. This suggests that CD45 is dispensable for beta-selection though p56lck is required.