Nomarski differential interference contrast microscopy (DIC) of lymphocyte surface morphology was combined with immunofluorescence studies on T and B cell markers on the thymus, lymph nodes, spleen, peripheral blood lymphocytes and thoracic duct lymph of female CBA mice. DIC identified smooth cells and several categories of villous cells; more extreme forms were present in lymph. Most B cells seemed to belong to the smooth group and most peripheral T cells to the villous group. Thymus cells were almost entirely smooth, but treatment with cortisone increased the proportion of villous cells to 50%. The surface morphology of lymphocytes was highly labile preventing direct identification or separation of T and B cells. In vivo removal of T cells by adult thymectomy, lethal irradiation and bone marrow reconstitution caused the villous cells to decrease. During recovery from irradiation, T lymphocytes tended to parallel villous cells, B lymphocytes smooth cells, but were differences between the spleen and lymph nodes. Mice deprived of T1 cells by adult thymectomy showed a modest decrease of smooth cells in the spleen and blood; mice depleted of T2 cells by anti-lymphocyte serum, or which were naturally deficient in T2 cells, were markedly lacking in villous cells. Thoracic duct lymph, which is rich in T2 cells, had a high proportion of extremely villous lymphocytes. Exposure to lymph induced extreme villous features in lymph node cells, and it was found that the thoracic duct lymph was markedly hypertonic to serum, although varying in osmolarity throughout the day. It is suggested that the villous shape of T2 cells is a circulatory adaptation, necessitated by the peculiar character of the lymphatic system in mice.