Surfaces of normal, cultured, and mitogen-stimulated mouse lymphoid cells have been examined by scanning electron microscopy (SEM). Lymphocytes with smooth highly villous and intermediate surfaces were observed in cell suspensions from both spleens and thymuses of normal mice and from spleens of congenitally athymic (nude) mice. Several strain-specific surface features were noted. Most striking of these was the spine-like appearance of microvilli on C57BL 6 lymphocytes. Although thymus cell suspensions contained somewhat more smooth cells than did spleen cell preparations, lymphocyte derivation could not be inferred from SEM examination. Studies of cells stimulated with agents mitogenic for thymus-derived lymphocytes (concanavalin A) or for bone marrow-derived lymphocytes (lipopolysaccharide) suggested instead that, in the mouse, development of a complex villous surface is a general concomitant of lymphocyte activation and transformation. © 1975.