Surface morphology of lymphocytes in nude mice and nu/+mice was studied by using a freeze fracture method. In nude mice, villous and intermediate cells were much more numerous than in nit/+mice, although smooth cells were also present in considerable numbers. The frequency of clustered intramembranous particles was low in glycerinated nude mice lymphocytes and stimulated lymphocytes of nu/+mice by PHA. Intramembranous particles were distributed somewhat more densely in nude mice lymphocytes than in nu/+mice and, on the contrary, nuclear pores were more frequent in nu/+mice than in nude mice. Both were more frequently seen in lymphocytes of nu/+mice stimulated by PHA. Our observation indicates that intramembranous particles may at least partly represent a protein and receptor site, that clustered intramembranous particles are well formed in mature T cells, and that nude mice lymphocytes have a tendency to show a villous surface morphology as compared with nu/+mice lymphocytes, while a considerable number of smooth cells in nude mice might represent at least partly precursor T and/or precursor B cells.