A sulfated cell surface glycoprotein with an apparent molecular weight of 185,000 is synthesized in the multicellular organism Volvox only during the limited period of embryogenesis. The lifetime of sulfate residues on this glycoprotein is very short (half-life about 20 min). Production of this sulfated glycoprotein sharply decreases to a minimum shortly before the onset of the differentiating cell cleavage--e.g., in asexual development, before the 32-cell embryo divides. It is demonstrated that the sulfated glycoprotein behaves in many respects as would the hypothetical cell surface component postulated by a recently published model [Sumper, M. (1979) FEBS Lett. 107, 241-246], which proposes an explanation for the cell-counting mechanism and the spatial control of differentiation that is operative in Volvox embryogenesis.