Intercellular invasion is the intrusion of one tissue into space occupied by a second tissue. The alternative situation to invasion, one characteristic of most coherent tissues, is segregation, with identifiable boundaries between contiguous tissues. Tissue segregation can be demonstrated as an active process by the sorting-out of the disorganized cell aggregates that are produced when mixed suspensions of dissociated cells are caused to reaggregate. In this situation, active reorganization of the disorganized aggregate restores a segregated tissue arrangement. Investigation of the sorting of the two principal tissues of the chick embryonic heart (namely, the myocardium and the mesenchyme) suggests an involvement of a fibronectin-rich interstitial matrix elaborated by the cardiac mesenchyme in the process of tissue segregation. During sorting the matrix appears selectively in regions occupied by the mesenchyme. Under conditions of culture non-permissive for matrix deposition, sorting fails to occur. Stimulation of matrix deposition by the addition of serum, transforming growth factor-beta, or isolated matrix itself is accompanied by sorting-out of the two tissues. The mutual invasion of the two tissues that occurs when mesenchymal and myocardial aggregates are fused occurs in regions where the mesenchymal matrix is lost. It is suggested that the presence of the fibronectin matrix in the cardiac mesenchyme promotes tissue segregation and organizational stability and that its loss establishes conditions in which invasion will occur.