On the basis of recently measured Rb-2 bound-state energies and contin
uum properties, we predict magnetically induced Feshbach resonances in
collisions of ultracold rubidium atoms. The resonances make it possib
le to control the sign and magnitude of the effective particle-particl
e interaction in a Rb Bose-Einstein condensate by tuning a bias magnet
ic field. For the case of Rb-85 they occur at field values in the rang
e where these atoms can be magnetostatically trapped. For Rb-87 they a
re predicted to occur at negative field values.