The stable arrangements of cells in tissues and organs require that most tissue cells, whatever their inherent abilities for active pseudopod-directed locomotion, remain confined to the boundaries of their parent tissues and organs for the lifetime of the organism. Contact inhibition of motility represents a potential mechanism for immobilizing cells in tissues and thus in promoting positional stability. In order to assess the effectiveness of contact inhibition in immobilizing cells in 3-dimensional tissue-like masses, spherical aggregates of chick heart, sclera and skin fibroblasts were fused with tritiated thymidine-labelled aggregates of the identical cell type. After being placed in contact, the 2 aggregates cohere and broaden the area of mutual contact to form a single aggregate with a planar interface between the two. Mobility of cells was detected by the migration of labelled cells across the interface and into the unlabelled aggregate. Based on measurements of distances moved in day-old aggregate pairs, it was found that fibroblasts moved up to 17 cell diameters and up to 36% as far in solid tissue masses as in monolayer culture under optimal conditions for directed migration.