The isolation by gel-permeation chromatography on Sephadex G-100 of a non-covalent complex of Cibacron Blue F3G-A (CB) with human serum albumin (HSA) is described. The complex presents a molar ratio of 3:1 CB-HSA and can be re-chromatographed under the same conditions without modification of its composition. However, complete dissociation occurs when the complex is chromatographed in the presence of denaturing agents. The effect of pH on the molar composition of the complex was also investigated by gel-permeation chromatography. Analogous complexes between CB and A and C cyanogen bromide fragments of unreduced HSA were also isolated by gel-permeation chromatography on Sephadex G-50. They present a molar ratio of 0.8:1 and 1.3:1 CB-protein for fragments A and C, respectively. These results suggest that two of the three molecules of CB bound to HSA may be located in the hydrophobic pocket corresponding to subdomain IIA, with the other molecule in the hydrophobic site corresponding to subdomain IIIA. The UV-Vis and dichroic circular spectra of the isolated complexes are reported.