Two novel heterobifunctional cross-linking reagents, which can be used to attach photoactivatable nitroaryl azides to primary amino groups of proteins, have been synthesized. The two compounds, N-5-azido-2-nitrobenzoyloxy-succinimide and ethyl N-5-azido-2-nitrobenzoylaminoacet-imidate-HCl, as well as ethyl 4-azidobenzimidate-HCl have been attached to lysine residues of cobra venom phospholipase A2 without a loss in enzymatic activity. Subsequent illumination of the modified forms of the enzyme at appropriate wavelengths under conditions in which the native enzyme exists in an aggregated state led to the formation of covalently linked dimers and large aggregates which could be separated by electrophoresis on polyacrylamide gels in the presence of sodium dodecyl sulfate.