Ultra performance liquid chromatography is a new development of the HPLC separation technique that allows separations on column materials at high pressures up to 10(8) Pa using particle diameters of 1.7 mu m. This increases the efficiency, the resolution and the speed of the separation. Four aqueous selenium standards were separated within 1.2 min on a 1.00 id x 50 mm reversed phase column in an ion-pair chromatographic system using a flow rate of 200 mu L min(-1). Hence, analysis times could be reduced to 1/10 compared with ordinary HPLC for aqueous standards. The precision and detection limits were comparable to values obtained by HPLC. Detection limits were better than 0.4 mu g Se L-1. A urine sample was analysed on a 1.0 id x 100 mm column within 5 min using a flow rate of 100 mu L min(-1). The improved separation efficiency, owing to the use of 1.7 mu m column particles, allowed the use of short columns. Hence, analysis times could be halved without loss of separation efficiency in this biological sample