An extension of the Standard Model by three singlet fermions with masses smaller than the electroweak scale allows to explain simultaneously neutrino oscillations, dark matter and baryon asymmetry of the Universe. We discuss the properties of neutral leptons in this model and the ways they can be searched for in particle physics experiments. We establish, in particular, a lower and an upper bound on the strength of interaction of neutral leptons coming from cosmological considerations and from the data on neutrino oscillations. We analyse the production of neutral leptons in the decays of different mesons and in pp collisions. We study in detail decays of neutral leptons and establish a lower bound on their mass coming from existing experimental data and Big Bang Nucleosynthesis. We argue that the search for a specific missing energy signal in kaon decays would allow to strengthen considerably the bounds on neutral fermion couplings and to find or definitely exclude them below the kaon threshold. To enter into cosmologically interesting parameter range for masses above kaon mass the dedicated searches similar to CERN PS191 experiment would be needed with the use of intensive proton beams. We argue that the use of CNGS, NuMI, T2K or NuTeV beams could allow to search for singlet leptons below charm in a large portion of the parameter space of the νMSM. The search of singlet fermions in the mass interval 2-5 GeV would require a considerable increase of the intensity of proton accelerators or the detailed analysis of kinematics of more than 1010 B-meson decays. © SISSA 2007.