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A new method is described for observing and quantifying an aspect of contact inhibition of cell movement that is sometimes called "contact paralysis". Based on the scanning acoustic microscope (SAM), the method can detect changes in the mechanical properties of cells, as well as changes in their motility and may therefore be more sensitive to some dynamic changes than methods based on optical microscopy. With this method intracellular motility of normal and transformed cells of epithelial and fibroblastic origin was investigated. By subtraction of SAM images patterns of motility, domains were detected that changed in a characteristic way among various cell lines. Wave-like and nucleating domains could be distinguished; they were also used for the quantification of motility. Like migration intracellular motility is influenced by cell-cell contacts. In zones where a cell touches its neighbours motility domains change or disappear depending on the cell type. In immortalized epithelial cells (XTH-2 cells) large quiescent zones developed in the region where contact with neighbouring cells was established, whereas in fibroblastic (3T3) cells motility was reduced less and did not change in domain pattern. Epithelial and fibroblastic cells were less motile when in contact with other cells or in confluent cultures than when solitary, i.e. their motility was contact inhibited. Transformed (SV40 3T3) cells, however, did not reduce their motility when in contact to or enclosed by other cells. The molecular basis for motility domains remains to be investigated.