Long-Evans Cinnamon (LEC) rats show a novel maturational arrest from CD4+8+ to CD4+8- thymocytes but a cause of this mutation is not identified. The candidate for this mutation is a defect in the function of CD4 or major histocompatibility complex (MHC) class II because gene-disrupted mice defective for CD4 or MHC class II molecules show a specific defect in CD4+ T cells. Previously, we showed that MHC class II is not a cause of this maturational arrest. Therefore, in this study, we focus on the function of CD4 molecules in LEC rat thymocytes. CD4 molecules on LEC rat thymocytes associated with protein tyrosine kinase, p56[lck], normally. Furthermore, cross-linking of CF4 molecules by anti-rat CD4 mAb elicited the elevation of intracellular calcium concentrations ([Ca2+]i) in LEC rat thymocytes, suggesting that CD4 molecules can deliver the signal normally. These results indicate that function of CD4 is normal and the maturational blockade of CD4+8- thymocytes in LEC rats is not caused by specific lymphocyte molecules that have been shown in gene-disrupted mice.