We have cloned and analyzed cDNA sequences derived from genes which encode the classical transplantation antigens. Our findings have led to a better understanding of the structure and function of these cell surface antigens, particularly with regard to their role in the presentation of tumor and viral antigens to the immune system. We have studied the expression and function of the human interleukin-2 receptor. Our findings suggest the existence of a secreted interleukin-2 receptor which can bind interleukin-2 efficiently. Furthermore, by using DNA-mediated gene transfer, we have demonstrated that the interleukin-2 receptor can function effectively in non-lymphoid cells.