IT has recently been shown1 that chick-heart fibroblasts in tissue culture influence each other's movement through their mutual contacts. In particular, it appears that when two fibroblasts touch, their further movement in the direction of their point of contact tends to be stopped. This phenomenon, which we called ‘contact inhibition’, is in principle capable of explaining many features of the behaviour of fibroblasts both in tissue culture and in the organism. Among other things, it implies a relative immobility within a group of fibroblasts, provided the group is enclosed by an external barrier of some sort, since the cells inhibit each other. Contact inhibition would therefore be expected to play a part in keeping the fibroblasts within an organism largely immobilized under normal conditions.