The yeast ribosome core particles partly depleted of acidic proteins, L44 and L45, were isolated and their activity was examined using a highly purified protein synthesizing system. It was shown that both the phenylalanine polymerization reaction and the elongation factor 2-dependent GTP hydrolysis were stimulated by the addition of the extracted acidic protein fraction. The results obtained clearly indicate a functional role of these proteins in the elongation step of eukaryotic protein synthesis. The essential parameters of ribosome reconstitution experiments are discussed.