Epitaxial ZnO films doped with Co were fabricated by the high throughput combinatorial laser molecular-beam epitaxy method. The solubility limit of Co in ZnO was determined. Transmission and cathodoluminescence spectra were measured to study the electronic structures of Co in Zn1-xCoxO films. Magnetoresistance behavior was measured for a selected sample co-doped with I mol% of Al to investigate s-d exchange interaction between the conducting s electron spins and the d electron spins localized at Co impurity. Huge magneto-optical effects comparable with that of Cd1-xMnxTe were observed on Zn1-xCoxO films. (C) 2002 Elsevier Science B.V. All rights reserved.