Peripheral blood lymphocytes (PBL) and alloreactive T cell lines of two male infants born to consanguinous parents and presenting with severe combined immunodeficiency (SCID) showed a pronounced deficiency in T cell activation. Although phenotypically normal, the proliferative response of the childrens' T cells was strongly reduced but could be improved by the addition of interleukin‐2 (IL‐2). Furthermore both childrens'T cells were unable to produce the cytokines IL‐2, interferon‐γ (IFN‐γ), IL‐4 and tumor necrosis factor‐α (TNF‐α). This multiple cytokine production deficiency could not be restored by IL‐2 or co‐stimulatory signals provided by antigen‐presenting cells (APC). Moreover, mRNA for IL‐2 and IFN‐γ could not be detected. In contrast, expression of the activation‐dependent cell surface markers CD25 and CD69 was within normal limits. To determine whether the functional defect of the patients' T cells was due to the absence or abnormal binding of transcription factors involved in cytokine gene expression, electrophoretic mobility shift assays were used to examine the DNA binding of AP‐1, Oct, CREB, SP1, NF‐ϰB and the nuclear factor of activated T cells (NF‐AT) to their respective response elements in the promoter of the IL‐2 gene. Whereas AP‐1, NF‐ϰB, Oct, CREB and SP1 displayed normal binding activities in nuclear extracts, the binding of NF‐AT to its IL‐2 promoter response element was barely detectable both before and after T cell stimulation. Our results strongly suggest that this NF‐AT/DNA binding defect is responsible for the multiple cytokine deficiency and the SCID phenotype observed in the two infant brothers.