  We define the chemical potential as the Lagrange multiplier of the baryon
charge operator in the transfer matrix formalism of QCD on a lattice.
Transforming the partition function into an euclidean path integral we get the
Hasenfratz-Karsh action both for Wilson and Kogut-Susskind fermions. In the
latter case the chemical potential in the spin-diagonal basis is half that in
the flavour basis. Some open problems in the spin-diagonal basis are pointed
out.
