Synthetic UV-detectable peptide pI markers were used to estimate isoelectric point (pI) values of proteins separated by capillary isoelectric focusing (CIEF) followed by cathodic mobilization in the absence of denaturing agents. The pI calculation and quantitative analysis of purified proteins showed the feasibility of these peptides as pI markers and internal standards in CIEF separation of proteins. Estimation of pI values of major proteins in human plasma was performed using the peptide pI markers, and the values were compared with those previously obtained by gel isoelectric focusing (IEF). Sera of immunoglobulin G (IgG) myeloma patients, which showed characteristic peaks of myeloma IgG in their CIEF patterns, were also subjected to the analysis and the pI values of the myeloma proteins have been estimated.