  We present the constraints on isocurvature fluctuations for a flat universe
implied by the Boomerang and Maxima-1 data on the anisotropy of the cosmic
microwave background. Because the new data defines the shape of the angular
power spectrum in the region of the first acoustic peaks much more clearly than
earlier data, even a tilted pure isocurvature model is now ruled out. However,
a mixed model with a sizable isocurvature contribution remains allowed. We
consider primordial fluctuations with different spectral indices for the
adiabatic and isocurvature perturbations, and find that the 95% C.L. upper
limit to the isocurvature contribution to the low multipoles is $\alpha \leq
0.63$. The upper limit to the contribution in the ${\l}\sim 200$ region is
$\alpha_{200} \leq 0.13$.
