Hybrid selection translation experiments have been carried out with genomic and cDNA relatives of two repetitive sequence families. On the basis of the in vitro translation products detected, it was found that transcripts complementary to these repeats are linked to several different mature mRNAs in stage 40 embryos of Xenopus laevis. One repeat hybridizes to mRNAs that direct the synthesis of 17 proteins. The second is present on mRNAs coding for 3 proteins. By estimating the abundance of these proteins among the translation products of total embryonic mRNA, it is inferred that all of the repeat bearing mRNAs are rare, less than one in 20,000 mRNA molecules.