AbstractInteractions in tissue culture of sensitized C57BL spleen and DBA/2 mastocytoma cells have been studied by microcinematography, electronmicroscopy and light microscopy. Motile lymphocytes with their characteristic hand mirror shape move on the surface membrane of the target cells, establishing a close contact within a few minutes. As seen by electron microscopy and equidensitometry, the contact between the cells is very firm, leading to damage of the target cell membrane while leaving the effector cell intact. Analysis of time‐lapse cinematography shows that one motile lymphocyte has the capacity to lyse more than one target cell, a phenomenon which may account for the highly efficient surveillance mechanismin vivo. The role of soluble factors and the importance of the lymphocytic uropod in cellular interactions are discusse