  We compute complete tree level matrix elements for $gg , q \bar q \rightarrow
b \bar b W^+W^-$. We analyze the irreducible backgrounds to top signal at the
Tevatron and at the LHC. Their contribution to the total cross section is about
$5 \%$ at the LHC, due to single resonant channels. Several distributions with
contributions from signal and backgrounds are presented.
