A bioassay to measure the incorporation of [14C]leucine into acid-precipitable polymers of suspension-cultured sycamore (Acer pseudoplatanus L.) cells is described. Using this assay, cell wall fragments solubilized from sycamore cell walls by partial acid hydrolysis are shown to contain components that inhibit the incorporation of [14C]leucine into the acid-precipitable polymers. This inhibition was not attributable to a suppression of [14C]leucine uptake. The effectiveness of the wall fragments in inhibiting [14C]leucine incorporation was substantially relieved by plasmolysis of the cells. Fragments released from starch and citrus pectin are shown not to possess such inhibitory activities.