Compact analytic expressions have been obtained for the first order perturbative QCD corrections to the inclusive spectra of the leptons in the semileptonic decays of polarised heavy quarks.<BR> Charmed and beautiful $\Lambda$ baryons from $Z^0$ decays can be viewed as sources of highly polarised charm and bottom quarks. Charged leptons and neutrinos from $\Lambda_b$ and $\Lambda_c$ decays can be used in the polarisation studies for the corresponding heavy quarks. Thus our results are applicable for the b quark polarisation measurements at LEP.<BR> Short lifetime enables polarisation studies for the top quark. The angular-energy spectra of the charged leptons are particularly useful in this respect whereas the distributions of the neutrinos are sensitive to deviations from the V-A structure of the charged weak current in the decay.