Influence of the different types of lateral discrete restraints on lateral-buckling resistance of slender steel beams are analyzed in this paper. For this particular problem, comparing to prestandards, new code for the design of steel structures, EN 1993-1-1, introduces modifications and detailed explanation of design methods . Investigations are performed on the slender castellated beams and three types of lateral restraints are considered: discrete restraint of the compression flange only, discrete restraint of the member in point on 1/5 of their height and discrete restraint of both flanges. For selected beams the ideal elastic critical buckling moment Mcr is determined first, using linear eigenvalue buckling analysis, and then nonlinear static analysis is performed (with geometrical and material nonlinearity). In the codified expression for the evaluation of critical buckling moment one can simulate only lateral-torsion restraints (pinned or some level of fixed conditions), so results in this paper show that correct interpretation of the lateral restraints are very important from the aspects of design accuracy.