The rejection of interferon alpha 1 gene-transfected mammary adenocarcinoma cells (TSA-IFN alpha) injected into syngeneic BALB/c mice was accompanied by an unusual stromal reaction and marked CD8-positive T-lymphocyte involvement. To investigate the biological background of this reaction, the possibility was evaluated that an interaction between TSA-IFN alpha and stromal cells might remodel the extracellular matrix (EM). When fibroblasts were co-cultured with TSA-IFN alpha or treated with exogenous IFN alpha, there was no change in their replication rate or collagen synthesis. By contrast, their fibronectin (FN) production and release were increased, resulting in enhanced fibroblast chemotaxis. These findings were mirrored by increased FN staining in the peritumoural and tumoural areas in vivo. IFN alpha thus determines increased FN production and hence massive local recruitment and activation of fibroblasts, with a modification of the EM. The several activities of IFN alpha should thus be considered prior to its employment in clinical trials.