  A diagrammatic technique is derived for calculating processes involving the
production of large numbers of particles. As an example, the amplitude of three
particles scattering into $n$ particles at the kinematical threshold in
$\lambda\phi^4$ theory is calculated. From the $3 \to n$ amplitude, it is seen
that the one-loop amplitude for $2 \to n$ processes is suppressed by a factor
of $1/n$.
