The use of argon-laser-induced fluorescence detection in sodium dodecyl sulfate-capillary gel electrophoresis (SDS-CGE) separations of proteins is described. A simple, rapid (10 min) precolumn labeling reaction for a series of standard proteins (molecular masses of 20100–77000) was developed which utilizes 4-fluoro-7-nitrobenzofurazan and 4-chloro-7-nitrobenzofurazan as fluorogenic labeling reagents. Detection limits of 25 ng/ml for labeled bovine serum albumin were obtained. These detection limits compare favorably with alternate detection techniques such as Coomassie blue staining (2.5 μg/ml) and UV-Vis absorption detection in SDS-CGE (0.5 μg/ml). Conditions were identified for the labeling reactions which produce no measurable zone broadening effects for labeled proteins relative to unlabeled species. By using relative migration data from labeled proteins and molecular mass calibration plots obtained exclusively from unlabeled proteins, molecular mass estimates of labeled proteins typically accurate to within 4.5% of literature values can be obtained. The labeling reaction thus maintains the molecular mass information inherent in the separation.