Rhabdomyosarcomas (RMSs) consist of a mixture of primitive mesenchymal cells as well as cells showing various stages of rhabdomyomatous differentiation. The qualitative and quantitative degree of the rhabdomyomatous differentiation of the cells, evaluated by their morphology and expression of defined structural and functional proteins, is accepted as the basis of diagnosis and is considered to be related to the biological behaviour of RMSs. Therefore we investigated solid experimentally induced murine RMSs, adherent (subconfluent, confluent) cell cultures obtained therefrom, and also suspension cultures and studied the expression of muscular differentiation markers (vimentin, desmin, myoglobin) and the formation of extracellular matrix components (fibronectin, laminin). When we compared solid tumours with adherent cell cultures of decreasing cell densities (confluent up to single cells) and with cells grown in suspension, we found a gradual decline of differentiation ("dedifferentiation"). This decline paralleled the decrease of cell-cell and cell-substrate contacts. In suspension cultures, cells were prevented from interacting with each other and the substratum, no rhabdomyomatous differentiation of the cells took place. If restoration of cellular contacts was allowed, either by adherent growth or by reinoculation into nude mice, the process of dedifferentiation was completely reversible. Consequently, it was demonstrated that the increase of cell-cell and cell-substrate contacts was strongly associated with the appearance or increasing expression of the desmin intermediate filament cytoskeleton and with formation of the extracellular matrix components fibronectin and laminin. The microfilament (F-actin) system was modulated from an impressive stress-fiber system in subconfluent to a dense network in confluent monolayers. The extent of cell-substrate contacts, mediated by extracellular matrix components, and the number of cell-cell interactions are responsible for the capability of a malignant mesenchymal cell, which is able to undergo rhabdomyomatous differentiation, to achieve the various stages of maturation.