Results obtained from the application of new preparatory methods for the scanning electron microscopy of cell suspensions have raised questions about the early report by Polliack et al., 1973 concerning the difference in surface structure between B derived and T derived lymphocytes. However, the question "how villous are B lymphocytes?" is regarded as still open in view of a series of recent reports in which the villous nature of B lymphocyte surface was confirmed by methods other than scanning electron microscopy (immunofluorescence studies with anti-smooth muscle antibody, cryofracture, immunofluorescence for surface Ig, as well as studies of live cells under the interference contrast microscope). It seems that the best way to clarify the problem of a possible surface difference between B and T derived cells is to develop a methodology which will permit the recognition of the presence of surface Ig on well-preserved lymphocytes under the scanning electron microscope. We plan therefore to adapt the method recently proposed by Kumon to the study of lymphocyte surface. The method includes the coupling of goat anti-human Ig with T4 bacteriophage, T4 being used as a visual surface marker recognizable under the scanning electron microscope. T4 will be coupled to goat gamma globulins with glutaraldehyde according to the method of Avrameas. The method will be applied mostly to normal human circulating lymphocytes and its value in the characterization of human leukemic lymphocytic cells will be studied. The significance of the project resides in the opportunity to study the surface structures of well-preserved lymphocytes in relation to the presence or the absence of surface Ig and at the level of each individual cell.