AbstractThe lymphocytes and serum of a nonidentical twin male with advanced colon cancer were evaluated in vitro for their ability to inhibit the growth of autologous tumor cells. The patient's serum enhanced autologous tumor cell growth. Three siblings including an HL‐A matched grade A female twin underwent similar studies. Benign lymph nodes of the twins and one sibling were evaluated for the ability of nodal macrophages to be sensitized to the patient's tumor antigen. The male twin patient received in‐tralymphatically in vitro sensitized nodal macrophages and lymphocytes from the female twin donor. Thirty days after treatment the patient's lymphocytes and serum inhibited the growth of autologous tumor ce