A modification of the leukocyte adherence inhibition test for the detection of cellular immunologic reactivity of mice to the mammary tumor virus (MTV) has been described. It involves the transfer of the leukocyte adherence inhibition factor (LAIF) produced by spleen cells from immunized animals when cultured with antigen to indicator cells, for which peritoneal exudate cells from normal mice are used. The method proves to be sensitive and highly reproducible. By crude separation of leukocyte subpopulations it became established that for the production of LAIF the following sequence is needed: 1. incubation of adherent cells with MTV;2. transfer of a soluble factor SF1 produced by the adherent cells to T cells; 3. transfer of another soluble factor SF2 released by T cells to the adherent cells.