A two-fold increase in polyadenylate [poly(A)] content occurs between fertilization and the two-cell stage in sea urchin zygotes. In this report the role of this cytoplasmic polyadenylation process in the provision of binding sites for poly(A)-associated proteins during early development of Lytechinus pictus is evaluated. Protein-associated poly(A) sequences, from ribonuclease-treated, post-mitochondrial supernatants of various developmental stages, were collected by nitrocellulose filtration and quantified by 3H-poly(U) complex formation. The proportion of protein-associated poly(A) rose from about 27% to about 60% of the total poly(A), on a nucleotide basis, during the period between fertilization and the eight-cell stage. However, the actual increase in number of poly(A) sequences associated with protein was more extensive, about 2.5-fold, since protein-associated poly(A) sequences average about 45 nucleotides longer than free poly(A). The protein-associated poly(A) of eggs and zygotes is found in two types of protease-sensitive complexes which sediment at 8--12 S and 15--20 S. The 8--12 S complex appears to be selectively increased in amount following fertilization. Sodium dodecyl sulfate-polyacrylamide gel electrophoresis of the poly(A) protein complex fraction indicates the presence of 87,000 and 130,000 molecular weight polypeptides in both eggs and zygotes. It is concluded that quantitative, but not qualitative, alterations in the proportion of protein-associated poly(A) accompanies post-fertilization cytoplasmic polyadenylation in sea urchin zygotes. The attachment of specific proteins to the 3' terminus of maternal RNA's may be involved in their subsequent activities during early embryogenesis.