Circular dichroism (CD) spectra have been measured in the aromatic region for recombinant human interleukin IL-1 beta and for site specific mutants in which each of the four tyrosines and the single tryptophan residue have been replaced one at a time by phenylalanine. These mutant proteins have been shown to have conformations that are closely similar to that of the wild type protein. By comparing the spectrum of each mutant with that of the wild type human protein it has been possible to assess the contribution of each aromatic residue to the CD spectrum of the latter. It has been shown that the spectrum is largely a result of contributions from Trp 120 and from Tyr 68.