Cultured skin fibroblasts from normal individuals and from patients with Duchenne muscular dystrophy spread equally rapidly when seeded on a glass substratum. Exposure to the ionophore monensin substantially suppresses normal and dystrophic fibroblast spreading in serum-free media for up to at least 100 min. Preincubation of normal fibroblasts with monensin causes a further reduction in cell spreading. Dystrophic fibroblasts fail to spread as well as normal cells after monensin preincubation. Such findings indicate that there is a delay in the secretion of functional adhesive surface proteins in monensin-preincubated normal fibroblasts and that this lag period is significantly longer in dystrophic fibroblasts. These data are consistent with findings of altered adhesive and secretory properties of fibroblasts from patients with Duchenne muscular dystrophy.