Intercellular adhesion is measured by a new method based on determination of the rates of attachment of single cells to confluent cell monolayers. The procedure is simple, rapid, and reproducible. Specific and nonspecific intercellular adhesions can be quantitated and distinguished from each other, and from the adhesion to glass (or plastic). The rate of adhesion of single cells to the monolayer is characteristic of more than 80% of the single-cell population. This method, therefore, provides a means for study of the molecular basis of intercellular adhesion.