Regulation at the post-transcriptional level is gaining significance at a rapid pace. One example is the storage of messenger mRNA molecules in a translationally quiescent state, the so-called "masked messengers". Their existence has been known since the 1960s, but many details of their composition and structure have not yet been resolved. Masked messenger RNAs are particularly abundant in the oocytes of the African clawed toad Xenopus laevis. The aim of this study has been to examine the proteins bound to stored mRNAs in the oocytes, by focussing on the Y-box proteins which had already been identified as major components in mRNA masking, and by analyzing some of the other unidentified mRNP proteins. The YB proteins were studied in greater detail, gaining fresh information about their RNA-binding properties, defining distinct binding domains. The presence of an mRNP-associated protein kinase was confirmed, and binding assays suggested that phosphorylation influences the ability of the YB proteins to bind to mRNA. cDNA expression libraries were screened both with an RNA-binding assay and with an immunoscreening method, isolating a variety of known and novel cDNAs. Peptide sequencing of mRNP proteins revealed the presence of an RNA helicase distinct from the translation initiation factor eIF4A. It is postulated that the RNA helicase, in addition to the YB proteins, will be seen to have an important role in the formation and activity of the masked messenger RNA particles.