Long monolithic silica-C-18 capillary columns of 100 mu m W. were prepared, and the efficiency was examined using reversed-phase HPLC under a pressure of up to 47 MPa. At linear velocities of 1-2 mm/s, 100000-500000 theoretical plates could be generated with a single column (90-440 cm in length) using an acetonitrile-water (80/20) mobile phase with a column dead time (t(0)) of 5-40 min. It was possible to prepare columns with a minimum plate height of 8.5 +/- 0.5 mu m and permeability of (1.45 +/- 0.09) X 10(-13) m(2). The chromatographic performance of a long octadecylsilylated monolithic silica capillary column was demonstrated by the high-efficiency separations of aromatic hydrocarbons, benzene derivatives, and a protein digest. The efficiency for a peptide was maintained for an injection of up to 0.5-2 ng. When three 100 mu m i.d. columns were connected to form a 1130-1240 cm column system, 1000000 theoretical plates were generated for aromatic hydrocarbons with retention factors of up to 2.4 with a t(0) of 150 min. The fact that very high efficiencies were obtained for the retained solutes suggests the practical utility of these long monolithic silica capillary columns.