  It is shown that the technique recently suggested by Lowell Brown for summing
the tree graphs at threshold can be extended to calculate the loop effects.
Explicit result is derived for the sum of one-loop graphs for the amplitude of
threshold production of $n$ on-mass-shell particles by one virtual in the
unbroken $\lambda \phi^4$ theory. It is also found that the tree-level
amplitude of production of $n$ particles by two incoming on-mass-shell
particles vanishes at the threshold for $n > 4$.
