Rhodamine-labelled alpha-actinin was specifically bound to actin containing filament bundles of demembranated fibroblasts, and was particularly associated with their termini. Optimal binding of rhodamine-alpha-actinin occurred at pH 6.0 - 6.2 and could be abolished by the addition of unlabeled alpha-actinin or myosin subfragment 1. The spatial relationships between the decorating alpha-actinin and several actin associated proteins was determined by double fluorescence microscopy.