We investigate transport through 5–10 nm thin epitaxial GaAs(001) and (110) barriers sandwiched between polycrystalline iron films. It turns out that tunnelling is the dominant transport mechanism; this is indicated by a nonlinear I–V-characteristic, an exponential dependence of the tunnelling current on the barrier thickness and the temperature dependence of the current. We observe a pronounced tunnelling magnetoresistance (TMR) effect at low magnetic fields. Annealing decreases the TMR effect drastically and indicates the sensitivity of this effect to layer intermixing. At high magnetic fields we observe a distinct negative magnetoresistance (MR) at low temperatures and a positive MR at higher temperatures. This negative MR contribution is only observed for ferromagnetic iron contacts and is absent if iron is replaced by copper or gold electrodes.