A single HT29 human colon adenocarcinoma cell was introduced into a fused-silica capillary and lysed, and the protein content was fluorescently labeled with the fluorogenic reagent 3-(2-furoyl)quinoline-2-carboxaldehyde. The labeled proteins were separated by capillary electrophoresis in a submicellar buffer and detected by laser-induced fluorescence in a postcolumn sheath-flow cuvette. Several dozen components were resolved. A number of experiments were done to verify that these components were proteins. Most components of the single-cell electropherogram had the same mobility as components present in the 30-100 kDa fraction of a protein extract prepared from the cell culture. One component was identified as a approximately 100 kDa protein by co-injecting the sample with purified protein obtained from an SDS-PAGE gel. Protein expression varied significantly between cells, but the average expression was consistent with that observed from a protein extract prepared from 10(6) cells.