The separation of riboflavin, flavin mononucleotide and flavin adenine dinucleotide was investigated by capillary zone electrophoresis using laser-induced fluorescence detection. In the systematic approach developed, the differential electrophoretic mobilities were first maximized by adjusting the pH. Increasing the buffer concentration improved the separation at the expense of migration times. A buffer consisting of 50 mM phosphate adjusted to pH 8.5 was found to provide a very efficient and stable electrophoretic system. Responses were linear within the range 0.1-100 mu mol L-1 and the detection limits of B-2 vitamers were 0.23 nmol L-1 or less. The method was successfully applied to a variety of biological tissues from different animals.