We have computed with a fine time grid the evolution of the elemental abundances of He, C, N and O ejected by young (t=30M_{sun}_) during their wind phase including the Wolf-Rayet phase and the ejecta from the core-collapse supernovae. We find that during the Wolf-Rayet phase (t<5Myr) the cluster ejecta composition suddenly becomes vastly overabundant in N for all initial abundances and in He, C and O for initial abundances higher than one-fifth of the solar. The C and O abundances in the cluster ejecta can reach over 50 times the solar value with important consequences for the chemical and hydrodynamical evolution of the surrounding interstellar medium.