We studied the fine structure of respiratory-tract cilia in three siblings with chronic respiratory disease, comparing them with those from a patient with Kartagener's syndrome who had dynein-deficient cilia and with control patients who had chronic bronchitis or chronic sinusitis. Electron microscopy of the siblings revealed a new abnormality in the ciliary axoneme--namely, lack of the radial spokes. Their cilia showed an eccentric central pair of tubules but otherwise had a normal central sheath, outer-doublet microtubules, nexin links and dynein arms. The cilia were immotile. Mucociliary clearance was completely lacking in the three siblings and in the patient with Kartagener's syndrome, but was normal in their parents and unaffected siblings. Sperm from the male sibling showed identical structural abnormalities and were immotile. We consider the radial spoke defect to be the congenital anomaly responsible for dysfunction of the mucociliary clearance mechanism in these three patients and of the immotile sperm in one of the them. This defect is apparently another cause of the "immotilecilia syndrome."