  We use cosmic microwave background data from WMAP, ACBAR, VSA and CBI, and
galaxy power spectrum data from 2dF, to constrain flat cosmologies based on the
Jordan-Brans-Dicke theory, using a Markov Chain Monte Carlo approach. Using a
parametrization based on \xi=1/4\omega, and performing an exploration in the
range \ln\xi \in [-9,3], we obtain a 95% marginalized probability bound of
\ln\xi < -6.2, corresponding to a 95% marginalized probability lower bound on
the Brans-Dicke parameter \omega>120.
