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 .