We have conducted a detailed study of the open clusters NGC 2451 A and B which are located along the same line of sight at 206pc and 370pc distance respectively. Although belonging to the nearest clusters they have not been much investigated until present due to strong contamination by field stars. ROSAT X-ray observations and optical UBVR photometry are used to identify cluster members by means of X-ray emission and colour-magnitude diagrams. For the first time the range of known probable cluster members has been extended to main sequence stars of spectral class M. Isochrone fitting yields an age of 50 to 80 Myrs for NGC 2451 A and about 50 Myrs for NGC 2451 B consistent with the X-ray luminosity distribution functions. Except from the occurence of four flares the stars of both clusters do not show strong long-term X-ray variability exceeding a factor 5 over a time span of 1 to 3 years. High-resolution spectra obtained with FEROS confirmed membershipof most of the X-ray selected cluster member candidates by means of radial velocity and allowed to obtain rotational velocities as well as metallicities and lithium abundances.