  Relativistic jets are streams of plasma moving at appreciable fractions of
the speed of light. They have been observed from stellar mass black holes
($\sim$3$-$20 solar masses, M$_\odot$) as well as supermassive black holes
($\sim$10$^6$$-$10$^9$ M$_\odot$) found in the centres of most galaxies. Jets
should also be produced by intermediate mass black holes ($\sim$10$^2$$-$10$^5$
M$_\odot$), although evidence for this third class of black hole has until
recently been weak. We report the detection of transient radio emission at the
location of the intermediate mass black hole candidate ESO 243-49 HLX-1, which
is consistent with a discrete jet ejection event. These observations also allow
us to refine the mass estimate of the black hole to be between $\sim$9
$\times$10$^{3}$ M$_\odot$ and $\sim$9 $\times$10$^{4}$ M$_\odot$.
