In this paper, active chain is introduced as a chain coded contour whose shape is changed during iterations while it stays closed, clockwise and 4 connected. The iterations of the proposed algorithm move the chain items toward the interior region. This behaviour is similar to the active contours (snakes). If the initial contour is counter-clockwise, the same algorithm causes the active chain to expand like a balloon and detect the inner boundaries of the regions. The chain coded contours of all the separate regions can be detected in one pass in O(NM) where N and M are the image dimensions in pixels. (C) 2001 Elsevier Science B.V. All rights reserved.