When molecules on the surface of a motile eukaryotic cell are crosslinked, they are moved to one end of the cell to form a "cap". This phenomenon, the process of which is called cap formation, was discovered in 1971 on lymphocytes and is a property of amoebae and all locomotory animal cells except sperm. The crosslinking is most easily achieved using a polyvalent antibody to a surface antigen on the cell. Cap formation can be visualised by attaching a fluorophore, such as fluorescein, to the antibody. Steps The antibody is bound to the cell. If the antibody is non-crosslinking (such as a Fab antibody fragment), the bound antibody is uniformly distributed. This can be done at 0 °C, room temperature, or 37 °C. If the antibody is crosslinking and bound to the cells at 0 °C, the distribution of antibodies has a patchy appearance. These “patches” are two-dimensional precipitates of antigen-antibody complex and are quite analogous to the three-dimensional precipitates that form in solution. If cells with patches are warmed up, the patches move to one end of the cell to form a cap. In lymphocytes, this capping process takes about 5 minutes. If carried out on cells attached to a substratum, the cap forms at the rear of the moving cell. Capping only occurs on motile cells and is therefore believed to reflect an intrinsic property of how cells move. It is an energy dependent process and in lymphocytes is partially inhibited by cytochalasin B (which disrupts microfilaments) but unaffected by colchicine (which disrupts microtubules). However, a combination of these drugs eliminates capping. A key feature of capping is that only those molecules that are crosslinked cap: Others do not. Cap formation is now seen as closely related to the carbon particle experiments of Abercrombie. In this case, crawling fibroblasts were held in a medium containing small (~1 micrometre in size) carbon particles. On occasion, these particles attached to the front leading edge of these cells: When they did so, the particles were observed to move rearward on the cell's dorsal surface. They did so in a roughly straight line, with the particle remaining initially stationary with respect to the substratum. The cell seemed to ooze forward under the particle. In view of what we know of capping, this phenomenon is now interpreted as follows: The particle is presumably stuck to many surface molecules, crosslinking them and forming a