A model has been devised to study the in vitro formation of desmonsomes. The model is based on the differential labeling of two subpopulations of a desmosome-forming human cancer line (C4I). The labeled subpopulations are dispersed, preincubated separately on a shaking water bath for 24 h to allow the internalization of desmosome fragments and the repair of the cell surface, and then mixed, and allowed to aggregate. Aliquots of the mixed suspension are fixed at various intervals. The time between mixing and fixation represents the maximum age of any junction between dissimilarly labeled cells. The beginnings of desmosome formation were observed within a few minutes after the beginning of aggregation. Close apposition of cell membranes was seen immediately after mixing, followed within 15 min by the appearance of a submembrane density in one or both of the interacting cells. Intracytoplasmic filament formation takes place at between 15 and 30 min. Desmosome formation is complete by 90 min. The process is accompanied by a progressive widening of the extracellular space and the desification and organization of the extracellular material and the submembrane plaques.