The adenosine deaminase (ADA) activities in blood lymphocytes of 41 normal children and 17 with recurrent respiratory tract infections were examined, and the T-lymphocytes of two children whose ADA activities were obviously lower than those of others were culturedin vitro. Then the exogenous human ADA gene was transfected into these cells by means of lipofectin mediated gene transfer. The results showed that the ADA activities in cultured T-lymphocytes were raised and the immunological were also improved.