  We study peculiarities of proximity effect in clean superconductor --
ferromagnet structures caused by either spatial or momentum dependence of the
exchange field. Even a small modulation of the exchange field along the
quasiparticle trajectories is shown to provide a long range contribution to the
supercurrent due to the specific interference of particle- and hole- like wave
functions. The momentum dependence of the exchange field caused by the spin --
orbit interaction results in the long -- range superconducting correlations
even in the absence of ferromagnetic domain structure and can explain the
recent experiments on ferromagnetic nanowires.
