A sulphated glycoprotein, denoted I-SG, has previously been shown to be synthesized in Volvox under developmental control [(1982) FEBS Lett. 143, 311-315]. It is demonstrated that I-SG synthesis is restricted to a period during embryonic inversion as short as 10 min (out of the 48 h life cycle). This glycoprotein was purified and chemically characterized. 62% of its amino acid residues are hydroxyproline. Clusters of hydroxyproline residues were found to occur in its polypeptide chain. Besides arabinose and galactose, 3-O-methylgalactose and 6-O-methylgalactose were identified as constituents of I-SG.