Chick myofibrils in different states of contraction were treated with fluorescein-labeled antibodies. The rabbit antibodies were prepared against chick myosin, light and heavy meromyosins, and actin. For any one state of contraction, a single myofibril was photographed through the phase contrast microscope, stained with one of the antisera, and photographed through the fluorescence microscope. The cytological changes in the sarcomeres accompanying contraction as observed under phase were correlated with changes in the distribution of the precipitated antibodies as observed under the fluorescence microscope. The changing patterns observed through the fluorescence microscope were compared with those predicted by the sliding filament model of contraction.