Although skeletal muscle thick filaments have been extensively studied, information on the structure of cardiac thick filarnents is limited. Since cardiac muscle differs in inany physiological properties from skeletal muscle it is important to elucidate the structure of the cardiac thick filament. The structure of isolated and negatively stained rabbit cardiac thick filaments has been analyzed from computed fourier transforms and image analysis. The transforms are detailed, showing a strong set of layer lines corresponding to a 42.9 nm quasi-helical repeat. The presence of relatively strong "forbidden" meridional reflections not expected frorn ideal helical symmetry on the second. fourth. fifth, seventh, eighth, and tenth layer lines suggest that the crossbridge array is perturbed from ideal helical synnnetry. Analysis of the phase differences for the primary reflections on the first layer line of transforms from 15 filaments showed an average difference of 170degrees. close to the value of 180degrees expected for an odd-stranded structure. Computer-filtered images of the isolated thick filaments unequivolcally demonstrate a three-stranded arrangement of the crossbridges oil the filairients and provide evidence that the crossbridge arranaenient is axially perturbed froni ideal helical syniuletry. (C) 2005 Elsevier Inc. All rights reserved.