A two-step zero-length crosslinking procedure for studying protein-protein complexes has been developed. One component of a complex is briefly incubated with 1-ethyl-3-(3-dimethylaminopropyl)carbodiimide (EDC) in the presence of N-hydroxysuccinimide resulting in the conversion of some of the protein carboxyls into succinimidyl esters. The reaction is stopped by addition of beta-mercaptoethanol and other interacting proteins are then added. Crosslinking arises from substitution of lysine epsilon-amino groups of these proteins for the succinimidyl moieties during a 1- to 2-h incubation period. The advantage of this method versus one-step zero-length crosslinking is that only one component of the complex is exposed to the crosslinker, which eliminates complications arising from the formation of crosslinks among several proteins of a multicomponent complex. Furthermore, crosslinks can be formed even in the presence of reagents, such as dithiothreitol and EDTA, that would interfere with direct crosslinking with EDC.