  We propose a microscopic description of heavy fermions in UPt_3. It is based
on the assumption that two of the three 5f electrons of the U ions are
localized. Band-structure calculations based on this supposition reproduce the
observed de Haas-van Alphen frequencies very well. The observed enhancement of
the quasiparticle mass as compared with the LDA band mass results from the
local Coulomb interaction of the delocalized f electrons with the localized
ones. Diagonalization of the Coulomb interaction matrix yields the level scheme
of the localized f^2 states. Assuming a splitting of the ground-state doublet
by the crystal field of similar size as in UPd_2Al_3 results in a mass
enhancement factor of order 10 in agreement with experiments.
