Abstract The excellent dissolving capacity of formic acid together with a propanol-2 gradient is utilized in a new system for reversed-phase high-performance liquid chromatographic separation of poliovirus polypeptides and a variety of large proteins. Differences in elution characteristics were detected between reduced and non-reduced proteins containing disulphide bridges as well as proteins modified at cysteinyl residues. The retention coefficients of single amino acids were used to calculate those of proteins. The correlation of calculated coefficients with actual retention times indicates that some proteins are bound via their full, unfolded length to the reversedphase support, whereas others partly preserved their secondary structure. Treatment of proteins with sodium dodecyl sulphate prior to injection dissociates these structural elements and leads to an increase in retention times. The high resolution of the system described should be applicable to the isolation and characterization of components of mixtures of proteins, particularly those of water-insoluble proteins of membranes or viruses, on the analytical and semipreparative scales.