  We report magnetic susceptibility $\chi(T)$ measurements on oxygen-isotope
exchanged La$_{1-x}$Ca$_{x}$MnO$_{3+y}$ up to 700 K. The $1/\chi(T)$ data show
that the ferromagnetic exchange-energy $J$ depends strongly on the
oxygen-isotope mass. The isotope effect on $J$ decreases with temperature up to
400 K and then increases again with temperature above 400 K. This unusual
temperature dependence of the isotope effect cannot be explained by existing
theories of the colossal magnetoresistance effect for doped manganites. The
present results thus provide essential constraints on the physics of
manganites.
