During early oogenesis in Xenopus laevis about half of the 5S ribosomal RNA and most of the transfer RNA produced is stored in a ribonucleoprotein (RNP) complex that sediments at 42S. The other half of the 5S RNA produced is stored in a separate 73 RNP particle. As ribosome production gets underway in mid-oogenesis both the 1|2S and 7S particles disappear, the 5S rRNA being incorporated into the ribosomes, the tRNA being released as a slower sedimenting particle. Heterogeneity in the composition of the 42S particle was observed with respect to both protein and RNA components. The proteins of the 423 particle, Mr 48000 (P48) and Mr 43000 (P43) appear to be cleaved to smaller proteins. A derivative of P43 of Mr 17000 (P17) possibly becomes a ribosomal protein. Several observations are consistent with the view that P43 may accompany 53 RNA to the nucleolus before its cleavage product (P17) becomes incorporated into the ribosome. The chemical and structural relationships of P48, P43 and the protein component of the 73 RNP particle, transcription factor IIIA were studied and it was established that they are the products of three different genes. The 423 particle proteins have also been shown to have a binding affinity for 53 RNA genes (P48), tRNA genes (P43) or ribosomal genes (P43). In vivo inhibition of 5S RNA transcription by an anti-P48 antibody or tRNA transcription by an anti-P43 antibody suggest that these proteins may have a role in the transcription of these genes. Interactions between P43 and tRNA genes were analysed in most detail and indicating a specific interaction between these two components. The results taken as a whole suggest a central role for 42S particle proteins in particular P43, in co-ordinating the formation of the protein translational machinary.