Using partially purified human leukocyte, and human tissue culture interferon, immunize 12 mice and 6 rabbits. Establish in laboratory suitable methods for detection and quantifications of antibody to human interferon, as well as to the other proteins present in the interferon preparations used for immunization. When one or more of the animals are judged to have adequate serum titers of antibody to interferon, the animal will be sacrificed and its spleen removed. Using the methods suggested in the report by Collins, Black, Strosberg, Haber and Eloch, in Proc. Natl. Acad. Sci., U.S., (Feb.), 1974, which involves transformation of spleen cells by SV40 virus, contractor will make his best efforts to develop many clones of cells derived from individual spleen cells. The tissue culture fluid from many such clones will be tested for the presence of antibody to contaminating proteins.