Cell separation techniques and scanning electron microscopy (SEM) were used to characterize the surface morphology of small lymphocytes in mouse bone marrow. Lymphocyte-rich fractions and unfractionated suspensions of bone marrow and spleen cells from 9--10-week-old C3H male mice were glutaraldehyde-fixed, syringed onto gelatin-coated silver membranes, dehydrated in ethanol, infiltrated with amyl acetate, critical point dried, coated with gold-palladium and examined by SEM. High proportions of cells were retained on the membranes. Purified spleen small lymphocytes showed unimodal distribution curves for cell diameter (mode, 3.4 micrometer) and for number of surface microvilli (mode, 55--60). Bone marrow small lymphocytes were identified initially in lymphocyte-rich marrow fractions and in erythroblast-depleted marrow from polycythemic mice as well as in normal whole marrow. The cells resembled spleen small lymphocytes in size distribution and they showed microvilli. However, the number of visible microvilli was lower on small lymphocytes in the bone marrow (mode, 35--40) than in the spleen. While in each small lymphocyte population the total number of microvilli was greater on larger cells than on smaller ones, the density of microvilli per unit area of cell surface tended to decrease with increasing cell size. The results establish that the small lymphocytes in mouse bone marrow, mainly locally-produced immature cells, have villous surfaces, but the number of microvilli per unit cell surface area is less than that on peripheral small lymphocytes, as seen in the spleen. Neither in the bone marrow nor in the spleen are subpopulations of small lymphocytes distinguishable solely by numbers of microvilli. The findings suggest that microvilli on bone marrow small lymphocytes may undergo further development during post-mitotic maturation, surface receptor expression and migration of the cells to peripheral lymphoid tissues.