The structural alteration of human fibrinogen (Fbg) by cis-diamminedichloroplatinum(II) (cis-DDP) was investigated using intrinsic fluorescence and circular dichroism (CD) spectra. In a preceding report, incubation of Fbg with cis-DDP under physiological conditions resulted in the cleavage of disulfide (S-S) bonds (N. Ohta, T. Yotsuyanagi and K. Ikeda, J. Pharmacobio-Dyn., 15, 611 (1992)). The intensity of fluorescence for cis-DDP-treated Fbg, in which 6.7 S-S bonds per mol of protein were cleaved, was reduced to 42% relative to native Fbg. CD studies suggested that the alpha-helix content decreased from 42% in its natural state to 29% in the cis-DDP-treated form. The extent of the fluorescence quenching and the decrease in helix correlated with the number of cleaved S-S bonds. The results indicate that cis-DDP leads to a secondary conformational alteration of Fbg involving a structural perturbation of nearby tryptophan residue(s) through S-S bond cleavage. In a buffer containing calcium, however, the smaller fluorescence quenching and the smaller helix decline were observed in cis-DDP-treated-Fbg, even though the same level of S-S cleavage occurred. Calcium binding would protect Fbg in terms of structural alteration as well as S-S cleavage by both dithiothreitol (DTT) and cis-DDP.