Indirect immunofluorescence has been used to study the distribution of fibronectin during the course of embryonic chick limb morphogenesis and differentiation. At all stages of development from 19 through 25, fibronectin is distributed throughout the non-differentiating mesenchymal tissue directly subjacent to the apical ectodermal ridge (AER), i.e. the mesenchyme extending 0.15 mm or so from the AER. Fibronectin is also distributed throughout the proximal condensing central core of the limb during the early stages of cartilage differentiation. In fact, fibronectin persists as a major component of the intercellular matrix in the central core of the limb following overt chondrogenic differentiation, since it is present as late as stage 27 throughout the well-differentiated cartilage rudiments of the radius, ulna, and humerus. The detection of fibronectin in differentiated cartilage is facilitated by pre-treatment of the sections with testicular hyaluronidase prior to immunofluorescent staining. In contrast to the chondrogenic central core of the limb, in the peripheral dorsal and ventral (myogenic) regions in which muscle differentiation is progressing, there is a progressive and striking diminution in fibronectin staining. By stage 27, little, if any, is present in the well-differentiated muscle primordia. Finally, at all stages of development, there is an accumulation of fibronectin at the ectodermal-mesenchymal interface, suggesting it is a component of embryonic limb basement membranes. On the basis of these observations, the possible role of fibronectin in limb cartilage and muscle differentiation and in other aspects of limb morphogenesis is discussed.