The HPGE-1000 apparatus (LabIntelligence, Menlo Park, CA) is a gel electrophoresis instrument with intermittent fluorescence scanning of the migration path and with preparative capability. An electroelution cup sealed with gel is placed onto the band of interest, identified and located under computer control, and the band is electroeluted into the cup at a right angle to the orientation of the resolving gel. The correct location of the eluted band and the degree of its recovery into the elution cup are then verified on the gel pattern, visualized on the computer screen. Using that procedure, SDS-conalbumin-FLUOS was electrophoresed at 5 V/cm in a discontinuous tricinate-chloride-Tris system at loads of 0.25 to 20 micrograms, using 5% agarose (MetaPhor, FMC), 0.03% SDS gel at 5 degrees C. The horizontal gel was partitioned at the sample loading slit between a gel in Tris-tricinate (prepared at the concentrations of an operative phase ZETA) and in Tris-chloride (prepared as phase BETA). The elution cup was sealed with the latter gel and overlayered with buffer of the composition of the former. This arrangement should provide for electroelution of the band as a highly concentrated stack. At electroelution times of 2, 3.5, 4-5, 12, 15 and 15 min at 15 V/cm yields were 58, 60, 54-76, 99, 99 and 84% for loads of 0.25, 0.5, 1, 4, 10 and 20 micrograms, respectively. At the most sensitive scale of detection (13), a full-scale peak was obtained at a load of 1.7 micrograms when the fluorophore (FLUOS, Boehringer-Mannheim) to protein ratio was 10:1. Similarly, homogeneous nucleosomal DNA (146 bp), electrophoresed in 0.2 x TBE buffer at a load of 5 micrograms, was near-quantitatively recovered into the same buffer by electroelution at 15 V/cm for 2.5 or 6 min.