The egg-yolk protein precursor, vitellogenin, is biosynthesized and secreted by Xenopus laevis (South African clawed toad) liver in response to a single dose of oestradiol-l7-3. The protein has previously been characterized as a Ca -binding glycolipophosphoprotein it is a dimer, mol. wt. 450,000-550,000, made up of two similar sized subunits. The biosynthesis of vitello- Benin has been studied using an in vitro liver slice system, concentrating particularly the posttranslational phosphorylation and glycosylation of the vitellogenin polypeptide. In pulse-chase experiments with (3H) leucine, cellular fractionation followed by SDS polyacrylamide gel electrophoresis has allowed the identification of 200,000 mol. wt. polypeptide precursors of vitellogenin associated with the microsomes. However, further experiments with (,'P) phosphate and (3H) sugars showed that the level of phosphorylation and glycosylation of the microsomal precursors is very low compared with secreted vitellogenin. A second distinct population of precursors has been identified in the post-microsomal fraction, but in contrast to the microsomal precursors, these are phosphorylated and glycosylated to a similar extent to secreted vitellogenin. The possible involvement of glycolipid intermediates during the glycosylation of vitellogenin has also been studied. It has been shown that oestrogen-treatment results in an increase in the formation of glycolipid intermediates, both in isolated microsomal preparations and in liver slices, and that these have properties consistent with them being of the dolichol phosphate sugartype.