The aggregation-dependent optical properties of eumelanin from human hair are examined. When aggregation is increased, the absorption spectrum extends to lower energy. The absorption spectra of oligomers isolated from black human hair and Sepia officinalis are comparable and are in quantitative agreement with the reported action spectra for photoinduced oxygen consumption and free-radical generation by eumelanin. The agreement between the optical properties of human hair and squid eumelanins suggests the fundamental molecular constituents of the pigments are similar and aggregation-dependent photophysical behavior is a general feature of all eumelanins.