High-resolution two-dimensional polyacrylamide gel electrophoresis shows that at least half of 370 denatured polypeptides from hamster cells and human cells are indistinguishable in terms of isoelectric points and molecular weights. Molecular evolution may have been more conservative for this set of proteins than sequence studies on soluble proteins have implied. This may be a consequence of complexities of intracellular organization and the numerous macromolecular interactions in which most polypeptides participate. It is suggested that the term "quinary structure" be used to refer to macromolecular interactions that are transient in vivo. Such interactions will not be evident from the composition of purified proteins, but they may constitute an important source of constraints on changes in primary structure.