The motility and deformability of lymphokine‐activated killer cells, purified by their adherence to plastic (A‐LAK cells), was investigated in vivo and in vitro. In vitro, A‐LAK cells were observed as intrinsically motile cells. They continuously changed their shape while forming protopods or pseudopods and crawled over the culture surface. A‐LAK cells were able to migrate across micropores of 3 μm diameter, which was three times smaller than the average diameter of an A‐LAK cell. Even in the absence of serum factors and of interleukin‐2 (IL‐2), more than half of the inoculated cells migrated across such micropore membranes within 18 h. Electron microscopic examination of these micropore membranes showed that the A‐LAK cells were highly deformable. A‐LAK cells also migrated across a confluent monolayer of endothelium‐like 10T1/2 cells. After 24 h incubation on the monolayers, about 20% of the A‐LAK cells were found underneath the monolayer. There, they actively moved in the narrow space between the monolayer and the bottom of the culture dish. In vivo IL‐2‐activated cells showing the large granular lymphocyte morphology accumulated in the hepatic microvasculature. Electron microscopic observations indicate that these cells did not accumulate by mechanical entrapment due to rigidity and size but rather by adherence to the endothelial lining of the sinusoidal capillaries. This also appeared to be the case for adoptively transferred A‐LAK cells, which were seen to be attached to the capillary wall. These observations stress the importance of adhesive entrapment to explain the accumulation of intravenously transferred LAK cells in the vascular beds.