The response of leucocytes to an intraperitoneal injection of sheep red blood cells was investigated in normal (+/+) mice, athymic (nu/nu) mice and their hybrids (nu/+). In +/+ mice (in addition to a non-specific decrease in the number of granulocytes in the first 6 h after injection) the leucocyte levels were below the initial levels almost throughout the experiment (12 days). This was due mainly to lymphocytopenia. In nu/+ and nu/nu mice a marked leucocytosis developed, due mainly to the increased numbers of lymphocytes. The response of lymphocytes estimated by the morphological appearance of nucleoli showed an increase in numbers of both active lymphocytes with RNA-synthesizing nucleoli and terminal lymphocytes with micronucleoli.