This paper reports on test bed and field studies to adapt and optimise two John Deere tractor engines for fatty acid methyl ester (biodiesel). Emissions were measured according to the international standard DIN EN ISO 8178-4, cycle C1, which is relevant for tractor engines. The results were compared to diesel fuel with and without optimisation of the engine for biodiesel. It could be shown that total particulate emissions did not change much with biodiesel but there was a strong increase in the soluble organic fraction while soot strongly decreased simultaneously. So in order to take full advantage of biodiesel the engines were also equipped with an oxidation catalyst. Compared to diesel fuel operation of the engines with oxidation catalyst, the emissions of hydrocarbons, carbon monoxide and particulates could be reduced with biodiesel whereas nitrogen oxides increased slightly. During a 600 h durability run and a tractor field test no fuel-related problems occured and a final engine inspection showed components still to be in an excellent condition. The results of engine oil analyses during both durability tests are presented.