  We investigate the constraints imposed by the first-year WMAP CMB data
extended to higher multipole by data from ACBAR, BOOMERANG, CBI and the VSA and
by the LSS data from the 2dF galaxy redshift survey on the possible amplitude
of primordial isocurvature modes. A flat universe with CDM and Lambda is
assumed, and the baryon, CDM (CI), and neutrino density (NID) and velocity
(NIV) isocurvature modes are considered. Constraints on the allowed
isocurvature contributions are established from the data for various
combinations of the adiabatic mode and one, two, and three isocurvature modes,
with intermode cross-correlations allowed. Since baryon and CDM isocurvature
are observationally virtually indistinguishable, these modes are not considered
separately. We find that when just a single isocurvature mode is added, the
present data allows an isocurvature fraction as large as 13+-6, 7+-4, and 13+-7
percent for adiabatic plus the CI, NID, and NIV modes, respectively. When two
isocurvature modes plus the adiabatic mode and cross-correlations are allowed,
these percentages rise to 47+-16, 34+-12, and 44+-12 for the combinations
CI+NID, CI+NIV, and NID+NIV, respectively. Finally, when all three isocurvature
modes and cross-correlations are allowed, the admissible isocurvature fraction
rises to 57+-9 per cent. The sensitivity of the results to the choice of prior
probability distribution is examined.
