In histological sections of s.c. transplanted V2 rabbit carcinoma, single tumor cells and small tumor cell groups were found at some distance from the main tumor mass. This led to the question of whether locomotion could represent a contributing factor in the invasiveness of the V2 carcinoma. The behavior of V2 cells was therefore recorded under experimental conditions of increasing complexity: on glass, on the surface of a normal explanted rabbit mesentery, and on and within mesenteries of rabbits which had received intraperitoneal implants of V2 carcinoma. Time lapse cinematography showed the locomotory activity of V2 cells to be unaffected by the different substrates. In all instances the carcinoma cells migrated singly, on the two plane substrates also in small groups, under production of large leading lamellae. Intraperitoneally implanted V2 cells, in addition to their migration on the surface of the mesentery, penetrated into the interior with continuation of their characteristic translocative motility. Although cell locomotion could be established as a mechanism in the invasiveness of the V2 carcinoma, we do not consider it to be the only relevant factor. Tumor cell proliferation and destructive effects of proteinases appear to be other mechanisms contributing to the functional complex of local spread.