  The two--loop mechanism of Bjorken and Weinberg is used to constrain flavor
changing neutral Higgs bosons. We calculate the complete set of two--loop
diagrams for the rare decay $\mu \rightarrow e + \gamma$ induced by such
neutral Higgs bosons, for arbitrary Higgs and top masses. The analytic result
is used to set limits on Higgs masses for some recent models with specific
ansatz about the flavor changing couplings. For example, in the Cheng--Sher
scenario of multi-Higgs doublet models, all neutral Higgs bosons possess flavor
changing ($f_i \leftrightarrow f_j)$ couplings proportional to $\sqrt{m_i
m_j}$. We find that the present limit on $\mu\to e\gamma$ implies that, in such
scheme, these neutral Higgs bosons should be heavier than $200$ GeV.
