The technique of adoptively transferring an immune response by injecting donor cells into irradiated recipient mice is based on the assumption that the host is rendered immunologically inert by irradiation. We present evidence that these mice are not inert; in the presence of donor cells of various types they mount a substantial immune response against both heterologous erythrocytes and dinitrophenylated bovine serum albumin. This restoration of the host requires donor cells for its expression and can account for up to 100% of the total response. The problems associated with this finding are discussed.