Antibodies to C-protein have been used to label fibres of glycerinated rabbit psoas muscle. The labelling patterns observed in the electron microscope show that C-protein is located on 7 stripes, spaced about 43 nm apart in each half of the A-band. Between two and four C-protein molecules per filament are present at each stripe. The labelling patterns also reveal the existence of two previously unrecognized components of the thick filament present in two further stripes in each half of the A-band. The location of C-protein and these other components accounts for many of the stripes seen in sections of the A-band and in negatively stained A-segments and provides new information on the packing of myosin molecules in the thick filament.