  The ratio of branching fractions of the decays $\Lambda^{0}_{b}\to
pK^{-}e^{+}e^{-}$ and $\Lambda^{0}_{b}\to pK^{-}\mu^{+}\mu^{-}$, $R^{-1}_{pK}$,
is measured for the first time using proton-proton collision data corresponding
to an integrated luminosity of 4.7 $fb^{-1}$ recorded with the LHCb experiment
at center-of-mass energies of 7, 8 and 13 TeV. In the dilepton mass-squared
range $0.1 < q^{2} < 6.0$ $GeV^{2}/c^{4}$ and the $pK^{-}$ mass range
$m(pK^{-}) < 2600$ $MeV/c^{2}$, the ratio of branching fractions is measured to
be $R^{-1}_{pK} = 1.17 ^{+0.18}_{-0.16} \pm 0.07$, where the first uncertainty
is statistical and the second systematic. This is the first test of lepton
universality with b baryons and the first observation of the decay
$\Lambda^{0}_{b}\to pK^{-}e^{+}e^{-}$.
