  This paper presents the construction of the Seiberg-Witten-Floer homology of
three-manifolds with non-trivial rational homology, and some properties of the
invariant of three-manifolds obtained by computing the Euler characteristic.
This new version corrects a mistake and some misprints that appear in the
published version (Internat. J. Math., Vol.7, N.5 (1996) 671-696). Various
sections are rewritten in more detail.
