Summary—A cell‐to‐substratum adhesion assay is developed to study the adhesion of sea‐urchin embryonic cells to coated substrata. The involvement in this process of both carbohydrate and protein molecules is reported. Concanavalin A (Con A) increases the attachment of cells to the substratum in a dose‐dependent manner and this effect is completely abolished when the incubation is carried out in the presence of the specific monocarbohydrate Con A‐inhibitor, α‐methyl‐d‐mannoside. A Con A‐mediated enhancement of cell‐to‐substratum adhesion was also detected on cells deprived of toposome, a glycoprotein complex responsible for cell‐to‐cell adhesion. The involvement of other molecules as well as toposome in the process of cell‐to‐substratum adhesion is also investigated. Results of thesein vitroexperiments indicate that all the molecules tested contribute to the proces