Synthetic UV-detectable peptide p/ markers were used to estimate isoelectric point (p/) values of proteins separated by capillary isoelectric focusing (CIEF) followed by cathodic mobilization in the absence of denaturing agents. The p/ calculation and quantitative analysis of purified proteins showed the feasibility of these peptides as p/ markers and internal standards in CIEF separation of proteins. Estimation of p/ values of major proteins in human plasma was performed using the peptide p/ 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 p/ values of the myeloma proteins have been estimated.