We present first results of the analysis of X-ray data (ROSAT PSPC and HRI pointings) and optical observations of the open cluster NGC 2451. This object consists of two different clusters A and B in the same line-of-sight at distances of 190 and 360 pc, respectively. NGC 2451A is one of the 10 nearest clusters, but has been little studied because of strong contamination by field stars. In order to improve this situation, we used X-ray observations to identify cluster members, a method proven to be very succesful at several other clusters. To identify the X-ray sources with optical counterparts we performed CCD photometry with the ESO 0.9m Dutch telescope. Altogether, 188 X-ray sources were found down to X-ray luminosities of a few 1028 erg s-1. Based on positional coincidence (i.e., offset < 25'') 122 of them could be identified with optical counterparts with magnitudes between V = 3 and 20, most of them ranging from 11 to 18. Of these 122 stars, 105 are possible members of clusters NGC 2451A or NGC 2451B, as derived from their position in the Colour-Magnitude-Diagram. Four stars showed strong flares during the X-ray observations, and we present a short analysis of the flare parameters. NGC 2451 A/B is especially interesting since it is a rather young (50 Myrs) and possibly metal-deficient cluster very close to us.