Trace amounts of enzymes within single human erythrocytes can be quantified by a combination of on-column reaction and capillary electrophoresis. A detection limit of 1.3 x 10(-21) mol of LDH was achieved with laser-induced fluorescence by monitoring the product of the enzyme-catalyzed reaction between lactate and NAD+. Single erythrocyte analysis clearly isolates the major forms of LDH. The variation of total LDH activity in a population of cells from a single individual is large, but the relative activities of the isoenzymes LDH-1 and LDH-2 are fairly constant. This can be explained by the distribution of cell age in the population. A lower enzyme activity is indicative of senescence. The efficient separation of different LDH forms and the high detection sensitivity opens up the possibility of multiple-enzyme assays with a single mammalian cell.