Diazepam (Valium/Roche), a benzodiazepine derivative, reversibly inhibited the spreading of chick embryo fibroblasts in a dose-related manner. This inhibition was accompanied by: 1 Fewer and less organized microfilament bundles. 2 A reduction in myosin fluorescence along these structures. 3 A reduction in the total myosin content of cells compared to corresponding controls. Overall results suggest that myosin plays an active role in the spreading of chick embryo fibroblasts.