Effects of low doses of cytochalasin B (CB, 2 micrograms/ml) and cytochalasin D (CD, 0.2 microgram/ml) on the spreading of normal mouse fibroblasts in culture were investigated. CB desorganized the cortical layer of actin microfilaments to cause partial or complete disappearance of microfilament bundles; focal contacts with the substrate seen by interference-reflection microscopy also disappeared. Low doses of CB did not inhibit the control, on the upper surface of these lamellas distal zones with convex outer edges ruffles were lacking. The disappearance of these ruffling active edges was accompanied with the loss of ability to clear the surface of lamellas from the concanavalin A receptors cross-linked by a corresponding ligand. Thus, ruffling active edges and focal contacts can be regarded as specialized parts of lamellas with increased sensitivity to cytochalasins; the presence of ruffling active edges is essential for the initiation of centripetal movement of the patches of cross-linked surface receptors.