Lymphocytes assume different plasmalemmal configurations in vitro with the following ultrastructural characteristics: (1) a change in general cell shape which is a departure from the usual spherical form; (2) the presence of three types of cytoplasmic extensions: pseudopodia, microvilli, and “specialized microvilli;” (3) the presence in areas contiguous to sites of altered plasmalemmal configuration of cytoplasmic microfilaments 70 to 90 Å in diameter. Specialized microvilli are slender, probably rigid, of variable length but with uniform diameters of 1000 Å, and usually arise from a localized area of the cytoplasmic membrane, characteristically in the region of the cell where organelles are concentrated. Unique to the lymphocyte, among hematopoietic cells, specialized microvilli may play a role in antigen recognition and immunologic cell-to-cell interaction. Cytoplasmic microfilaments are interpreted in this report to represent the lymphocyte’s contractile machinery making possible cell locomotion and changes in plasmalemmal configurations.