Freeze-etch electron microscopy of pure RecA protein aggregates, as well as of RecA protein complexes on single-stranded and double-stranded DNA formed with various nucleotides, has permitted a clearer discrimination between the two different helical polymers that this protein forms. Both are continuous, single-start, right-handed helices; however, the form observed when ATP or non-hydrolyzable ATP analogs are present has a pitch of 9.5 nm and a diameter of 10 nm, while the other form, observed in the absence of ATP or its analogs, or in the presence of ADP, has a pitch of 6 nm and a diameter of 12 nm. The former "long pitch" helix is found only when RecA protein is bound to DNA. The latter "short pitch" helix is also observed in pure RecA protein polymers (also termed rods) and in the needle-like paracrystals of RecA protein that form in the presence of magnesium or spermidine ions, representing bundles of rods closely packed in register. Addition of ATP or non-hydrolyzable ATP analogs in the absence of DNA dissociates the pure RecA protein crystals, as well as individual helical rods, into short curvilinear chains of attached monomers. These chains typically form closed, circular rings of 7(+/- 1) protein monomers, similar in construction to a single turn of the RecA protein helix, but significantly broader in diameter. The role of ATP in interconverting the various polymeric forms of RecA protein is discussed within the context that ATP functions as a reversible allosteric effector of RecA protein, much as it mediates reversible conformational changes in other vectoral motor proteins such as myosin, dynein, kinesin and the 70,000 Mr "heat shock" ATPases. We discuss how cyclic conversions back and forth between the short- and long-pitch conformations of RecA protein could mediate in reversible single-stranded and double-stranded DNA interactions during the search for homology.