Side chain amino groups of microbial Poly(epsilon-L-lysine) (epsilon-PL) were chemically modified with an amino labeling reagent, 4-dimethylaminoazobenzene-4'-sulfonyl chloride (dabsyl chloride) to give dabsylated epsilon-PL (Dabs-epsilon-PL). Aggregation behavior of Dabs-epsilon-PL in aqueous DMSO solution was studied through UV-Vis and CD spectra. In DMSO solution, Dabs-epsilon-PL shows a symmetric visible absorption at 450 nm, corresponding to the azobenzene chromophore in the monomeric state. Upon addition of water to the DMSO solution, the lambda(max) is shifted to around 400 nm. CD spectra were detected in the region of the blue shifted band: a pair of CD signals of opposite sign appears with a crossover point related to the shifted lambda(max). Similar trends were also observed for UV-Vis and CD spectra of dabsylated oligo(epsilon-L-lysine)mers. These spectroscopic results indicate that side chain azo chromophores of Dabs-epsilon-PL form H-aggregates and are disposed according to a chiral arrangement with right handedness in aqueous DMSO solution.