  We study Abramsky's applicative bisimilarity abstractly, in the context of
call-by-value $\lambda$-calculi with algebraic effects. We first of all endow a
computational $\lambda$-calculus with a monadic operational semantics. We then
show how the theory of relators provides precisely what is needed to generalise
applicative bisimilarity to such a calculus, and to single out those monads and
relators for which applicative bisimilarity is a congruence, thus a sound
methodology for program equivalence. This is done by studying Howe's method in
the abstract.
