Human lymphocytes of known B or T derivation were examined by scanning electron microscopy (SEM) before and after rosetting with SRBC. After collection of the cells onto silver membranes the samples were prepared for SEM by the critical point drying method. Sheep RBC frequently underwent sphero-echinocyte transformation and multiple projections extended from their surfaces. This was readily noticeable after storage of SRBC in the cold and washing in Hanks, but more prominent after rosetting. These erythrocyte surface alterations were less apparent when freshly withdrawn cells were used. Spontaneous sheep erythrocyte rosettes (E-R), a marker for human T lymphocytes, were prepared with normal peripheral blood lymphocytes (PBL), thymic cells, and cultured T cells. EAC-rosettes (EAC-R), used to identify B lymphocytes with complement receptors, were prepared with normal PBL and cultured B cells. The majority of rosetting T lymphocytes had generally smooth surfaces while about 20% had an intermediate number of microvilli and 15% were more villous and indistinguishable from villous B cells. Studies of rosetting thymocytes and cultured T cells however indicated that the surface of some T cells alters on rosetting, becoming more villous and thus account for the higher numbers of villous T cells seen in E-rosettes. Point to point contact sites between SRBC and T lymphocytes were more frequent than broad zones of attachment. The majority of rosetting B lymphocytes had multiple microvilli, about 25% had a moderate number of microvilli and less than 10% had smooth surfaces similar to those of most T cells. Areas of contact between EAC and B lymphocytes were frequently broad zones of attachment. The study confirms that in many cases B and T lymphocytes can be distinguished by their surface architecture as seen under the SEM; however, about 20% of rosetting B and T cells have similar surfaces with intermediate numbers of surface microvilli and cannot be distinguished by SEM without parallel immunologic identification.