The characteristics of human lymphocyte motility and its relationship to the redistribution of surface membrane antigens (capping) are poorly defined. Since chronic lymphocytic leukemia (CLL) cells cap poorly when compared with normal human lymphocytes, this study was undertaken to compare the motility of these two cell types. A modification of the Boyden chamber system was employed to quantify lymphocyte motility by placing lymphocyte suspensions on 8-mum convoluted-pore nitrocellulose filters and measuring the depth of migration of the cells into the filter at 37 degrees C. After 3 hr of incubation, CLL cells migrated significantly less into the filter than normal cells. Incubation in the presence of sodium azide or at 4 degrees C abolished all motility, indicating the active nature of the process. The relative motility of individual CLL patients' cells correlated best with the proportion of abnormal cells present as determined by surface receptor assays. The possibility that decreased cell motility in CLL was a reflection of enrichment by a “bone marrow-derived” (B cell) population was eliminated by the finding that normal B cells purified by gradient separation of rosetted cells migrated faster than normal T cells and considerably faster than CLL cells. Motility of normal and CLL lymphocytes was decreased by cytochalasin B and increased by colchicine, vincristine, and vinblastine. Thus, human lymphocyte motility appears to be dependent on microfilament integrity but not to require the colchicine-sensitive cytoskeleton. The decreased motility of CLL cells is the result of an intrinsic cell abnormality, but this finding cannot fully explain the decreased capping, since in human lymphocytes the latter is not prevented by an inhibitor of motility.