We present an electron microscopy study on the polymerization state of recA in the absence of DNA. In solution recA exists as monomers, small complexes not clearly longer than wide (approximately 9-18 nm), and filaments (diameter approximately 11 nm and variable lengths). We have attempted to quantify the relative amounts of these species by length measurements of the particles on electron micrographs. The percentages of each of these types was found to depend on recA concentration, temperature and presence in the incubation mixture of Mg2+, ATP gamma S, salt or D2O. These additives do not have an absolute effect on polymerization but rather shift the polymerization equilibrium of recA (which depends on recA concentration) up or down the concentration scale.