To determine whether p56lck protein tyrosine kinase and CD45 protein tyrosine phosphatase are involved in the signal transduction during intrathymic differentiation of gamma/delta T cells, we have examined the development of T cells expressing V gamma 3 T cell receptor (TCR) in mice deficient for either protein. The skin from both mice contained significantly reduced numbers of dendritic epidermal T cells expressing decreased levels of V gamma 3 TCR at the cell surface. Analysis of the fetal thymus from these mice suggested that maturation of V gamma 3 thymocytes was blocked at the immature stage that was characterized by the low level of V gamma 3 TCR and the high level of heat stable antigen. These results imply that both p56lck and CD45 are involved in the signal transduction during maturation of V gamma 3 T cells in the fetal thymus.