Monolithic ion exchange CIM® (Convective Interaction Media) columns have been proven in quantitative analysis of different immunoglobulins such as IgM and IgG from human plasma or cell supernatants. The separation mechanism is based on ionic interactions between the ion exchange monolith and immunoglobulin that are controlled by salt concentration. Here we present another possibility of IgM determination based on monolithic CIM® OH columns where the interactions may be controlled by changes in salt concentration or by pH increase. A method for quantitative HPLC determination of IgM in cell supernatant with fluorimetric detection was developed on CIM® OH column (0.34 mL) by means of pH increase. Optimal separation of IgM from cell supernatant matrix was achieved by combining acetate and phosphate buffer in a suitable gradient profile. Two different quantification methods, i.e. calibration curve and standard addition.