Nanoparticles with a mean hydrodynamic radius of 16 run and low polydispersity (P.I. = 0. 1) were spontaneously formed by the complexation of poly(ethylene oxide)-block-poly(L-glutamate) (PEO-b-PLGlu) with diminazene. Only one of two possible binding sites of each diminazene molecule was involved in complexation. As determined by UV-vis difference spectra measurements. the complex binding constant is on the order of 1-2 x 10(4) M-1. Circular dichroism measurements showed that the highly water-soluble diminazene can induce and stabilize the cc-helical secondary structure of a PLGlu block.