An understanding of the three-dimensional structure of a biological macromolecular complex is essential to fully understand its function. A component tree is a topological and geometric image descriptor that captures information regarding the structure of an image based on the connected components determined by different grayness thresholds. We believe interactive visual exploration of component trees of (the density maps of) macromolecular complexes can yield much information about their structure. To illustrate how component trees can convey important structural information, we consider component trees of four recombinant procapsids of a bacteriophage (cystovirus ϕ6), and show how differences between the component trees reflect the fact that each non-wild-type mutant of the procapsid has an incomplete set of constituent proteins.