Depending on the surface immunoglobulin of the lymphocyte membrane, lymphocyte has been classified either as a surface immunoglobulin bearing cell (SIBC) or a non-SIBC. An ultrastructural approach of the freeze-etched lymphocyte membranes of normal subjects revealed the existence of micro-processes and small particles on its surface. The density of micro-processes on the lymphocyte membrane was distributed in the range from 0.3 to 6.8/μ(2), and its histogram showed two peaks at 0-1 and 3-4/μ(2). The lymphocytes of normal subjects, in whom SIBC was in 18 percent, were freeze-etched, and 28 percent of the lympbocytes showed higher density of micro-processes more than 3/μ(2). After the filtration of the lymphocytes through a tetron fiber column, the percentages of both SIBC and the lymphocytes showing the micro-processes more than 3/μ(2) were decreased simultaneously. In acute and chronic lymphocytic leukemias, the examination of lymphocyte subpopulations and the membrane architectures of the leukemic lymphocytes revealed the similar tendency of membrane property as observed in non-SIBC and SIBC of normal subjects.